Tires - Nitrogen air loss study

Filling tires with nitrogen rather than air is becoming a common practice in the replacement tire market. This service offers tire dealers another avenue for making money while also promoting safety. The claimed safety benefits often include the potential for reducing air loss compared to an air-filled tire. Maintaining proper inflation can help prevent tire overheating; promote optimum tread life; and reduce rubber aging and wheel corrosion. The use of nitrogen in large truck fleets and the commercial tire industry are well documented and support these claims.

The National Highway Traffic Safety Administration (NHTSA) has seen reduced aging of tires filled with nitrogen. Though the data does support that passenger car tires could benefit by all the claims made for nitrogen, tire manufacturers say that they already design tires to perform well with air inflation. And while nitrogen will do no harm, manufacturers say that they don't see the need to use nitrogen, which generally adds $5 or more per tire charge.

Consumer Reports wanted to find out if nitrogen is worth the price, so we purchased a Nitrogen Inflation System and checked out how well the inflation held up over a one year period. We evaluated pairs of 31 tire models of H- and V-speed rated, all-season tires used in our tread wear test from 2006. We filled one tire per model with air and the other with nitrogen. The test was quite simple: fill and set the inflation pressure at room temperature to 30 psi (pounds per square inch); set the tire outdoors for one year; and then recheck the inflation pressure at room temperature after a one year period.

The tires were filled and deflated three times with nitrogen to purge the air out of the tire cavity. We also used an oxygen analyzer to be sure we had 95-percent nitrogen purity in the tire--the claimed purity limit of our nitrogen system, which generates nitrogen gas from ambient air.

The test started on September 20, 2006 and the final measurements were taken on September 20, 2007.  The results show nitrogen does reduce pressure loss over time, but the reduction is only a 1.3 psi difference from air-filled tires. The average loss of air-filled tires was just 3.5 psi from the initial 30 pressure setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial 30 psi setting. More important, all tires lost air pressure regardless of the inflation medium, so consumers should check their tires' air pressure routinely. No evaluation was done to assess the aging claim.

Bottom line: Overall, consumers can use nitrogen and might enjoy the slight improvement in air retention provided, but it's not a substitute for regular inflation checks.

--Gene Petersen

Added 10/11/07:

There has been great interest in this topic, as evidenced by the comments. Among the user posts are many questions—more than could be addressed in a simple comments format. We have posted a "Nitrogen in Tires Q&A" directly answering many of these questions.

Read the latest tire Ratings and advice. Discuss Tires in the Consumer Reports forums.

Comments

[Peter Vongboupha]

Were the tires installed on a vehicle? Would the weight of a vehicle have a greater effect on the loss of psi?

[Howard]

Seems to me that since the difference is so small and nitrogen is difficult to get and more costly that it is not worth the effort and the cost.

[William A. Tomb]

Were the tires on new rims or older ones?

[Brad]

I think the bottom-line comment about being sure to check pressure regardless of air or nitro is key. I think the tendency would be to NOT check nitro filled tires as frequently as air filled.

[Janet Givens]

I have nitrogen in the tires on my Prius and noticed an immediate improvement in gas mileage of 3-5 mpg. Better gas mileage was the selling point when the dealer suggested the change. I'm interested to know why gas mileage was not part of the test.

[Tuan]

I agree with all of the comments above. It is a best practice to regularly maintain a car. Many people will get the oil change and for the most part, it ends there. Skipping scheduled maintenance may save money in the near term, but the long term wear on a car that is not serviced can lead to much more expensive remedies.

Car tires should be visually inspected before one drives their car.

[Kyle]

I just thought I'd remind everyone that Nitrogen makes up like 75-78% of ambient air, so air verse Nitrogen should make little or no difference.

[Chris]

Good Point!!!I hadn't looked at it that way, Think I'll skip the Nitrogen

[John]

I just had Nitrogen added in our 99 9-3. I'm getting roughly 1 mpg better than before. I also noticed a difference in the ride. It seems there is less give and therefore thighter steering. My wife who didnt' know about the nitrogen made the same observation about the handling. Anyway, seems worth it to me.

[Jason]

The test implies the tires simply sat outside for a year and were not driven or used during that time. What good is that? Tires get used. Without testing the impact of actual wear and tear this test is useless. Why not fill a long term test vehicle with two nitrogen tires and two air tires and then drive it for 10k miles and inspect for wear, PSI, etc?

[mark]

I agree with the other post that the test is flawed because it leaves doubts in people's mind, what if the tires got actually mounted to the car and driven around as they should be. Will the results be the same?

On the other hand, air contains 80 % of nitrogen already, and we are paying $5 a pop to increase that to 95%? Why?

[Norman]

I use ordinary air in my Prius, but spent $40 on nitrogen for my wife's 2007 Camry hybrid. Within a few months she flattened two tires on road trash and destroyed a third while trying to evade a pothole. My 2004 Prius, which has never had a flat, surpasses without nitrogen the mileage of other Priuses I have checked. Is there a message here? I suggest that driving technique may do more for fuel economy than nitrogen.

[David]

I think we are missing some of the advantages here. First, the air loss mentioned above is 2.2 vs 3.5 psi. That is a significant difference, even at this low inflation pressure. Also, nitrogen is an intert gas, and so will react with the rubber/chemical compounds much less, contributing to reduced wear. Another point is that nitrogen will not heat up like oxygen, so during extended highway driving you will reduce the overinflation and wear/tear resulting from heat build-up.

[Brad]

This seems like a flawed test because the tires weren't tested under "normal operating conditions." CR wouldn't test mattresses for excessive sag just by leaving them in the lab for a year or evaluate a car's gas mileage by running it on a treadmill. Ride comfort, tire handling and tread life and even braking distance could all be different for air- vs. nitrogen-filled tires. This "research" is irresponsibly shallow.

[Patrick]

May I suggest that improvements seen in MPG and ride comfort by using nitrogen are a result of the placebo effect? If you get N for "free", such as at Costco, then fine, enjoy your green valve caps, but paying money for the stuff is like throwing it into the 80% nitrogen wind!

[WIlliam Stackhouse]

What about daily, weekly, and/or monthly fluctuations due to significant changes in the ambient air temperatures? While PSI LOSS (from Oct to Oct when ambient air temperatures would be expected to be consistent) was comparable between Air and Nitrogen filled tires, would you expect that the PSI VARIATION due to ambient air temperature changes over shorter periods of time could be much more pronounced? Also, the question that was posted by someone else about the heat build-up during sustained highway driving is a related question that should be answered.

[Steve Sasaki]

If you don't want to give any credence to this test that's fine. Enjoy your proven increased mpg, handling, and "ride comfort" (See anecdotal evidence). But there are physical laws that we abide by here on earth and one of them is called the Ideal Gas Law. It simple states that N2 and "air" are pretty much the same thing and that they both share the same characteristics in a tire such as thermal expansion, molecule size, etc. CO2 also falls under this law. I don't know if I'm going to start hearing many bubbles bursting but if you don't want to believe me then believe the tire companies. They do have some smart people working there. I assume one day green valve caps will be a sign of ones gullibility.

As far the corrosiveness of O2 in air. You do have a point there. Luckily, tires wear out years before the effects of O2 would ever show any signs. There are other factors way more damaging to tires than air: UV, road hazards, poor maintenance, dog pee, the curb, my wife...

[Chris M]

The respondents that stated better fuel economy, must not have had proper inflation to begin with. PSI is PSI. 95% Nitrogen or air. As long as the PSI is the same, fuel economy would be the same. 95% Nitrogen would be lighter than air but it would not be enough to make a difference in fuel economy as long as PSI is the same.

[Steve Motta]

A flawed study and analysis.

And sadly quiet on the advantages of using nitrogen in heavy trucks where 18 tires need to be maintained weekly to pressures of 100psi.

Please consider a second, more complete look at this.

[Chris M]

The respondents that stated better fuel economy, must not have had proper inflation to begin with. PSI is PSI. 95% Nitrogen or air. As long as the PSI is the same, fuel economy would be the same. 95% Nitrogen would be lighter than air but it would not be enough to make a difference in fuel economy as long as PSI is the same.

[Darren Stukes]

If nitrogen is free sure why not. If I was planning to reuse the carcases of my old tires as they do commonly in the Heavy Duty industry- lets talk. This should not even be an issue if your 3,000 mile interval oil changes include checking your tires- if not I would look elsewhere for service. Other than that- Buy yourself $2 air gauge and check your tires when you get gas. Nitrogen doesn't prevent flat tires- and the last time I checked 1 PSI didn't make the difference when the guy next to me on the freeway had a blowout from a FLAT tire he was driving on.

[Jeff Bartlett]

There has been great interesting in this topic, as evidenced by the comments. Among the user posts are many questions--more than could be addressed in a simple comments format. We have posted a "Nitrogen in Tires Q&A" directly answering many of these questions.

Click on my name below to visit the post, also at:
http://blogs.consumerreports.org/cars/2007/10/nitrogen-tires-.html

[David Scribner]

We have seen a study by a major vehicle manufacturer which revealed most air pressure loss in tires does not take place through the sidewall of the tire but is due to tire bead to rim bead seat and flange leakage around the valve stem and valve core areas. I dont think any type of gas used would improve pressure loss in these cases.

The Rubber Manufacturers Association has also issued this statement:

Applications such as aircraft, mining, and commercial/heavy use utilize nitrogen to help reduce the risk of internal combustion (fire) if the brake/rim/wheel components overheat. Also, dry nitrogen is used in professional racing to help reduce variation in inflation pressures (caused by moisture) where even small differences in pressure can affect vehicle handling at the extreme limits of performance.

For normal tire service applications, nitrogen inflation is not required. However, nitrogen inflation is permissible as its properties may contribute to minor reductions in inflation pressure loss. Nevertheless, several other sources of pressure leaks, such as punctures, tire/rim interface (bead), valve, valve/rim interface, and the wheel, may negate the benefit of nitrogen.

If the tire inflation pressure is below the pressure specified on the vehicle placard, the tire must be re-inflated – whether with air or nitrogen – to the proper inflation pressure.

RMA warns that depending on nitrogen alone to reduce the requirements for inflation maintenance may, in fact, lead to under inflated operation, which may result in premature tire failure.

“With the right amount of inflation pressure, you will achieve optimum tire performance,” RMA wrote. “This means your tires will wear longer, save fuel and help prevent accidents.”

And above all, the RMA bulletin strongly reminded motorists to check tire pressure at least once a month when tires are cold and to use the vehicle manufacturer’s recommended inflation pressure.

“Whether inflated by air or nitrogen, regular inflation pressure maintenance remains critical and necessary,” RMA wrote. “Use of nitrogen alone is not a replacement for regular inflation pressure maintenance.”

The RMA bulletin notes that nitrogen is an inert (non-flammable) gas – basically, nothing more than dry air with oxygen removed (air contains about 78% nitrogen). Because of its inert properties, nitrogen is often used in highly specialized service applications and/or demanding environments.

[Note - CR shortened this message for space, though did not alter content.]

[Frank Ruscitti]

My question to Consumer Reports

When filling the Air Inflated Tires was the air source using a Refrigerated Air Dryer or some type of Moisture Dryer?

Most comsumers fill there tires with Compressed Air that has not been filtered and I personally have seen my wifes Volvo lose 2+ Pounds per month and have upto 10 PSI Pressure increase on a hot day in AZ.

With Nitrogen I see almost No Pressure lose with Monthly checks and only a 4 PSI Increase.

This gives the vehicle a very much improved ride and handling.

I paid $29.95 for the inflation and feel that If I receive only a 10% better tire life that pays for the Nitrogen.

The ride and handing improvement is more then worth the cost.

[Mat]

As many have already pointed out, this report is flawed in that it is not reflective of actual real life conditions where tires under continual driving conditions lose pressure much faster. As a long time subscriber to CR, I am disappointed by the incorrect inference it may lead to. But, as the report’s bottom line points out there is no substitute for checking tire pressure regularly.

Four point to make:

1. Various studies point to higher average pressure loss (about 1-2 psi per month). One Bridgestone study says air inflated tires lost an average of 2.7 psi per month and nitrogen inflated tires lost an average of 0.7 psi per month. (Guy Walenga, Clemson Tire Conference, March, 2004)

2. Despite repeated advice from consumer and industry groups, fact is, consumers simply do not check tire pressures regularly (you can forget about monthly checks). People just don't have the time and that is unlikely to change soon. When did you check your tire pressure last? According to studies, a large majority of us (~90% in a report by RMA http://www.goodyear.com/media/pr/nat_2003/22655ti.html ) do not check our tire pressure properly. So, many of us are likely riding on underinflated tires. If Nitrogen tire inflation can reduce that underinflation effect, it contributes, even if modestly, to improved gas mileage and ride comfort. Mileage improvement stories listed here are likely factual and these people, like many others, were riding on underinflated tires and nitrogen tire inflation helped, just like air inflation would have. But with nitrogen, proper inflation may last just a little longer.

3. Even CR test does shows ~38% improvement in pressure retention with nitrogen. If average tires lose 1-2 psi per month, compounded difference over 4-6 months may be well over 4 psi.

4. Another perspective is its impact on energy conservation and climate change. In that regard, individual paybacks become less important than overall contribution to reduced energy consumption. Examples are energy saving light bulbs and hybrid cars with long paybacks.

5. There should be no doubt on whether nitrogen tire filling is better or not. It definitely is. Question should be if it’s worth the price charged? May be not for everyone. It should be looked as a matter of convenience. If you do not have to top off tires as often, is it worth it the charge?

[Ed]

This report does not give a true picture of the advantages of using nitrogen in tires. It is typical of running tests and drawing conclusions under the wrong conditions.
Before I used N, I would fill each tire to factory specs before each long (4000 Miles +) trip, and would invariably have to refill them again after the trip. If I forgot or neglected to do this, the tires wore prematurely on the outside edges due to underinflation brought on by tire pressure loss. This did not happen with my properly filled N tires.

Right now I am still driving on a N refill I got in Olean, NY in the fall of 2006. I have put on 18,000 miles having driven twice to the East Coast, once to the West Coast including two trips to Las Vegas. I reside in Colorado at an altitude of over 6000 ft. where the winter temperatures regularly drop to sub zero temperatures and I drive to altitudes of over 10,000 ft. Not once have I had to add N since this N refill in NY, nor has my pressure varied more than a pound or so through extreme heat and cold conditions. My very sensitive tire pressure system (2006 factory)has remained "silent" throughout all this time, even at 20 degree below zero temperatures.

One other factor the report fails to address properly is that under conditions of driving long and fast under very hot conditions, an air-filled tire's pressure will increase significantly, possibly even approaching dangerous levels. This does not happen with N filled tires. Check out why race cars use N in their tires.

I am not a dealer nor do I operate any kind of tire service. I am retired, and have now experienced at least 10 continuous years of driving with N filled tires. The few bucks spent on getting my tires filled with N is insignificant compared to the extra milage and even tire wear that I have since enjoyed.

[Christopher K]

As a Manager of Vehicle Services for Bridgestone/Firestone, I have had my fair share of questions regarding niotrogen for inflation of tires. As a corporation, BFRC does not condemn nor condone nitrogen for use of inflating their tires. I work in a corporate store, and we do not offer nitrogen as an option to our customers. According to my bosses, the added expense of equipment and increased cost to the consumer versus benefits to the consumer does not warrant the use of nitrogen in tires.

In my 20+ years of automotive experience both as a manager and a technician, I have noticed the average consumer pays little to no attention to their tires until they go flat, or they are worn to the point that the consumer experiences reduced traction due to worn tires.

A tires inflation should be checked monthly, at minimum, according to the RMA. Tire pressure cannot be accurately checked by just looking at a tire. Modern radial tires will have an inherant bulge to them, especially on the front tires, where the majority of the weight is, namely the engine and transmission. the ONLY ACCURATE WAY TO TEST FOR PROPER INFLATION IS TO USE A RELIABLE TIRE PRESSURE GUAGE.

A tire under normal circumstances will lose 1 PSI per month due to permeability. In addition, a tire will gain or lose 1 PSI per 10 degree Farenheit change in temperature. This is why you should check your tires when they are cold. A cold tire is defined as a tire that has not been driven on for more than 1 mile, or one who has sat for 4 hours or more.

According to the RMA, if you must check tires while hot, add 4PSI to the cold inflation pressure without exceeding the maximum pressure on the sidewall of the tire.

If consumers would simply maintain their tires at proper inflation pressure the US could save 4 million gallons of gasoline per day! Unfortunately, the average consumer only checks their tires every 3 months or 3000 miles when they get their oil change.

Please, check your tires, your fuel economy, safety, handling and tire life is at stake!

[Craig j Knarich]

I have just received a call from a good friend about nitrogen and how his buddies are busting on him about the use of nitrogen. First let me state that I have a duel life in the working mans world. I am a state certified engineer in hydraulics in the fire service. I am a professional fire fighter. Second I own by industry standards the most advanced tire company nationwide. PIT CREW TIRE SERVICE I have been in the tire business since 1981. My company was one of the first to go with nitrogen in the states. January 2001 when I built my new truck I thoroughly researched the gas and reviewed its results in Europe. It had been used in Europe since the 1990's the difference is I wanted to do my own study so I invested in a $10k industrial grade nitrogen generator.(AIR PRODUCTS) I used it for 2 years without telling my customers what I had done never charging them a single dime. My blind study showed that my customers enjoyed the ride of there car better and they started wondering why the tires lasted longer and had improved gas mileage for those freaks that really watch there fuel economy. And yes I had a few. How do I know this, I call every customer after I put tires on there cars. Yes not the norm but I can tell you first hand it works. My company does not charge for nitrogen you purchase tires I give it free. It is a cost of doing business. I will tell you I would never pay for nitrogen in my tires I have seen what the bean counters are doing and it is going to bite them right in the BUTT. nitrogen generators must be serviced once a year. They must have the prism membranes and the o2 sensor changed at a cost of around $500 annually and they are not doing it. Plain and simple nitrogen is like bottled water was in the 80's want to buy some bottled water and the laughed you out as they went to the tap. Now nobody drinks tap water that is why I don't sell it but be warned there are people out there that are unknowing selling air at nitrogen prices and I have seen it first hand. Warning if you buy it ask them to show you a o2 gauge to prove that they are adding n2 as long as your tire obtains a 95% or better concentration or the o2 sensor gauge reads 4.0 or better you are o.k. Air is 78% nitrogen 21% oxygen and 1% other gasses. NEWS FLASH they tell you the reason oxygen seeps through the betel sidewall because the oxygen is smaller and the n2 molecule is larger that is false and every reporter gets it wrong along with the sales men n2 is smaller o2 is larger the n2 molecule is very dense and the o2 molecule is not that is why it seeps through and that is why you get a better ride and improved mileage. The tire also will stand taller and be more rigid and have reduced tire squirm and hydro planing in rain Second the people in southern states will benefit the most and will notice the difference more. The only people that will not notice a difference are those people that are driving on run flat tires there are many more thing I could debate about this but there is only so much time I will tell you yes it works and the tire dealers should give it for free when you purchase tires from there store.

[HAROLD HERZLICH]

Nitrogen inflation of tires is the next paradigm shift in tire maintenance. Rubber oxidizes and the expensive antioxidant chemicals incorporated during manufacture, while being effective, are consumed or lost during tire service. The opportunity to inflate with an already proven oxygen free, less permiable inert gas that helps the tire retain it's pressure longer while virtually eliminating the inflation oxidation component is something the industry should encourage rather than taking a neutral position.

Under inflation is a major contributor to tire over heating, injury, vehicle loss of steering control, loss of wet traction and tire failure.

TPM devices are not going to eliminate marginal under inflation misuse. In fact these devices may encourage it by having the motorist believe that tires don't have to be checked until the device triggers (at 20% underinflation) when it's already weakened the tire and caused an increase in fuel consumption.

While one psi advantage may seem small, fuel consumption increases by about .33 percent/psi.

Rather than the micro effect on an induvidual, environmentally responsible CR reporting should include the macro carbon footprint benefit of nitrogen inflation for the billion tires that are delivering 3 trillion miles of service per year.

Environmentally responsible people know the single aluminum can they recycle, will by itself have no measurable benefit, but when the macro effect of milions of people acting together, the benefit is massive.

Your readers are smart enough to crunch the numbers and be amazed by how much fuel is saved by an even small improved nitrogen inflation pressure advantage.

TPM devices will benefit from high purity nitrogen inflation with fewer false alarms,less marginal untriggered under inflation and other damage.

Nitrogen inflation will improve a vehicles fuel performance. it will not make a tire bullet proof. Some claims being made are not technically sound and are actually irresponsible, hurting the concept.

CR should amend it's report and extrapolate its inflation differential findings to fuel efficiency on a billion tire/three trillion miles per year macro scale showing the many millions of gallons of fuel saved. It is reported that under inflation wastes an estimated 600 million gallons ($1,800,000,000 at $3.00/gallon) per year. How can we conclude that nitrogen inflation is too expensive???

Why doesn"t the Department of Energy get involved?

[Greg]

Another recent CR article states: "What’s more, all tires tend to leak over time. Pressure loss averaged 6 psi for 36 all-season models we checked after one year." It is located at: http://www.consumerreports.org/cro/cars/tire-maintenance-1205/index.htm

How does that study differ from this study? Why almost double the difference (3.5 lb to 6 lb) in air pressure loss?

[Joseph Ruan]

Your getting ripped off and thinking your getting better gas milage is your imagination. I worked as a tow truck operator 15years and have been a tire manager for the last 8years and the air you breath is 75% (3/4) nitro. Joe the Tire Guy.

[DJlargo]

Air is already 80% nitrogen. And its free. If you want to pay for 95% nitrogen be my guest.

[EWordmon]

Darn! DJlargo and Joe the Tire Guy beat me to the punch. This ought to be on Penn & Teller's show.

Air is already 80% nitrogen. Hey, but if you want to pay $20 for 95% nitrogen by all means...

Most likely the gas mileage improvement is due to slower pressure loss over time. Harold hits the slower pressure loss above. The savings are negligible.

Go for the low hanging fruit. Make sure you change your oil. Use a lower viscosity oil if your owner's manual indicates you can. Make sure your air filter is clean and changed often. Ensure that your car has had a recent tune up. Check belts and/or serpentine belt.

And most important, check your tire pressures often. This will negate any comparison with air and 95% nitrogen and is a good practice anyway... ;-)

[Keane]

For the most part, if you are driving a normal car and not for racing, I would think that air is good enough considering the fact that it already has 78% nitrogen.

The key is to check the tire pressure regularly, at least once a month for vehicles and once a week for motorcycles.

~100% nitrogen filled tires can only help that much, it still depends on you to be discipline enough to check the tire pressure on a regular basis.

[TangoCharlie]

Reality check on nitrogen.

1. "Believe Ideal Gas Law. It simply states that N2 and "air" are pretty much the same thing, so both will act the same."
Fact: Incorrect. Fact is that Oxygen in the air permeates (leaks) out FOUR times faster than nitrogen. That makes nitrogen retain tire pressure for longer time.

2. Nitrogen "Drivers should check and refill tires regularly - monthly, if not weekly."
Fact: Most of us DO NOT check tire pressures regularly. At most, it is visual inspection, which can be very inaccurate. For many people, refilling tires less often with nitrogen is a convenience which may be worth $20.

2. "Air is already 78% nitrogen, what good will it be to get >95% nitrogen"
Fact: It is the OXYGEN that is bad. Oxygen leaks out of tire 4 times faster than nitrogen and also contributes to rubber damage. That is why bags of potato chips are filled with nitrogen - to preserve it. With nitrogen tire filling, bad oxygen is reduced by >75%.

3. "Industry spent millions of dollars on TPM systems. They will warn of low tire pressure anyway"
Fact: TPMS will light up only when pressure goes below 25% of set pressure. That is 8 psi in a 32 psig tire. At that point, cars will have lost >5% extra in fuel besides having measurable negative safety impact.

4. "Savings with nitrogen are miniscule and economically unattractive to consumers"
Fact: If ~3% (10c/gallon) are minisule for average driver, yes, the savings are miniscule. Regarding individual payback, you may also consider not buying energy saving devices like CFL light bulbs, more expensive energy saving appliances and hybrid cars because they all have multi-year payback.

[Tom]

Take it from an engineer. One common claim made by tire dealers is that nitrogen-filled tires do not react as much to temperature as air-filled. This claim is false! All gases change volume essentially the same amount due to temperature changes. (read a thermodynamics textbook) The difference between air and nitrogen is so small you could never measure it with a tire guage. So remember to always check your tires because even if tires didn't leak at all seasonal temperature changes will affect tire pressure enough to make a significant difference.

[Steve S]

I'm going to address just one point - the effects of the differences in leak rates for oxygen and nitrogen through a tire. Assume that the leak rate of nitrogen is one-forth that of oxygen, as some posters have claimed. Further, take the example of a tire that, when inflated with air, loses 5 psi per month. Since the leak rate of oxygen is much faster than nitrogen, and since the tire is filled with air which is approximately 20% oxygen (that is, contains 4 times as much nitrogen as oxygen), the tire in it's first month will lose equal amounts of nitrogen and oxygen.

Then, the tire is brought back up to the correct pressure using plain ol' air. But now, the tire contains more than 80% nitrogen because the added air was mostly (80%) nitrogen.

The effect of this, over several months, is to gradually increase the percentage of nitrogen in a tire. In this example, the tire reaches 90% nitrogen in 4 months, and it stabilizes at about 94% nitrogen after 12 months.

Hence, in this example, you get much of the (supposed) benefits of nitrogen just by the normal process of keeping your tires topped off at the correct pressure.

The percent nitrogen in the air that the tires stabilizes at is the same, and is only dependent on the relative leak rates of oxygen and nitrogen. The time (number of months) that it takes the tire to reach this point is dependent on the absolute leak rate through the tire. The slower the leak rate (for both oxygen and nitrogen), the longer to reach the equilibrium point.

[pmk]

Let me start by saying that I've been a tire engineer for 15 years.

No tire company inflates their tires with N2. It's been tested, it's a joke. Rolling tires don't lose more air through the innerliner than tires sitting against a wall. If anything, they may lose air due to tires slipping on the rim during accel/decel, but that would further squelch the differences seen in the CR study. Save your money for a good gauge and use it often.

[Robert Kershaw]

It seems there is a lot of discussion on the benefits, or lack thereof, when using nitrogen for tire inflations. No one has mentioned the hazard of using pure nitrogen.

For a homeowner with a nitrogen generator in an open garage, the danger is probably very low. But, if a tire shop replaced the air compressor with a nitrogen compressor the breathing hazard could be very real. Humans require a minimum of 10% oxygen to remain conscious. Less than 5% oxygen is usually fatal. Using pure nitrogen in a tire repair area where inflation, breakdown, re-inflation occurs could result in death.

The petrochemical industry has used nitrogen for decades and has developed very strict rules for handling, connecting and working around nitrogen. BREATHING PURE NITROGEN IS FATAL.

If you use nitrogen please consider the hazard. Never inhale pure nitrogen. Ensure adequate ventilation is present. Place appropriate warning placards around the nitrogen generator/compressor or use area.

[Chris Fogerty]

Some are saying they get better gas mileage with the nitrogen in their tires. Did they have proper pressure before you changed to nitrogen???

[Darrell]

Nitrogen, intake 'turbinators', fuel system 'rare earth' magnets, fuel 'catalysts', oil additives, pricey 'high power'/ultra low resistance spark plug wires, spark plugs with split electrodes, etc all have one thing in common. The separate fools from their money make and con artists rich. Properly maintained vehicles that use high quality fuels and fluids will not benefit from any of these modern snake oils. But if it makes you feel better, then go for it.

Added note re nitrogen and prevention of rubber deterioration not mentioned in previous posts: The most significant rubber deterioration occurs on the outside of the tire where ozone and UV light from the sun are the major agents causing damage. Take a look at the inside of a 5 year old tire freshly removed from a wheel and note the the rubber is smooth and looks new. Now look at the external side wall. Put another way, tires rot from the outside in. Nitrogen inside the tires will not prevent this external damage.

[David Smith]

I vote for nitrogen. I have a Convaid stroller for my daughter. The Convaid is like a wheelchair. The 8" front tires are quite small, and carry a lot of weight. I had to pump them up quite regulary. Then I found out the local Costco inflated their tires with nitrogen. They graciously pumped up the tires of the stroller. I noticed that the tires maintain their pressure longer, and I have to pump them up only about 1/4 as often. I didn't do a scientific study/keep track of dates and pressures, but my experience is enough to convince me that filling tires with nitrogen has merit.

[David Smith]

A suggestion for CR in a follow-up study: repeat the study with pressure monitors on the tires. Log the pressures as vehicles drive in different conditions and report the results. CR could also look at other factors such as road surface quality, style of driving etc. Putting air and nitrogen tires on the same test vehicle would eliminate many independnt variables.

[Beejal]

I have a pair of Goodyear Allegra passenger type tires that benefited quite a bit from inflation with nitrogen vs. air. When I inflated them with air, they seemed really susceptible to pressure fluctuations (both up and down) when I checked them 1-2 times per month. Also, the ride was very uncomfortable (hard and noisy) and the handling was loose compared to the OEM Bridgestones on my Mitsubishi Galant.

I was very disappointed with my tires, but after hearing about nitrogen inflation, I decided to give that a go. After inflating the Goodyear passenger type tires with nitrogen, my ride is much quieter and more forgiving over the rough roads here. The handling did indeed "tighten up." Finally, the pressure fluctuations stopped... I still check the pressure in my tires monthly, and over the course of 14 months, I've only had to adjust the pressure in my tires once (vs. 5-6 times with air-filling).

My fuel economy is the same since I previously monitored and adjusted the pressure in my tires frequently. However, I still feel like the nitrogen is a worthwhile investment, as it also improved ride and handling for me. I think the relative lack of water vapor in nitrogen vs. compressed air is another reason for the advantages.

[Travis]

As a mechanic I think the two biggest problems with nitrogen (other then it being a scam) are.

You normally can't get it at a smaller garage or quick lube, so you have to go to a tire dealer every time your tire is low.

And the bigger problem is no one ever checks thier own tires. They expect someone else to do it. And if they are bringing it to a shop without nitrogen they probably aren't doing it either. Not just because they don't have nitrogen but, because the last time someone with nitrogen filled tires caught them checking thier tires they got yelled at. So after one or two oil changes of not checking the tires the ones with nitrogen will be under inflated and the ones with normal air will be right where they are supposed to be.

I don't know how they are marketing this stuff but, you would think that any body who paid extra to have more constant tire preasure with 95% nitrogen would be willing to let it get dilluted down to 94% or 93% so that they are actually at the proper PSI.

[John Palazzini, Woodland Hills CA]

I would also think that there would be alot less oxidation with a nitrogen filled tire. Neoprene oxidizes over time and will have its physical properties compromised. So, with Nitrogen, there should be less oxidation and hence, less degradation.

[Mike Demers]

A few missing bits of information. The temperature/pressure factor is influenced by the amount of water vapor in the tire. The water vapor is the reason for pressure differences with temperature changes. The required 95% purity of N2 in the tire assures that most of the water vapor has been removed. A static test of tires is not a true test, a tire in use on a vehicle goes through many temperature changes, which changes pressure. The higher pressure from high temperatures should cause more laekage. Tires inflated with N2 are not as susetable to huge pressure increases/decreases caused by temperature. Remember--even 1% underinflation will decrease fuel mileage and tire life. The new Nissian GTR is reported as having N2 in the tires from the factory.
Key ingredients for proper tire inflation: 95% purity and monthly pressure checks.

[Doug]

First of all, it is not Nitro, it is Nitrogen, they are completely different topics.

The amount of difference Nitrogen has made in my 99 BMW M3 is, absolutely nothng, but then again, I monitor tire pressure very regularly, just like my oil level & everything else about the car or the SUV I drive sometimes, or any vehicle I've ever owned. Everyone should.

I monitor tire pressures, and I use my car for driving to work, at High Speed Driving Events, and at a race track for the occasional track day. I have thoroughly tested the difference between running Nitrogen & plain old air on the track & on the street & guess what? There is no difference after trying it in multiple setes of tires, none, nada... None of my tires wore out from the inside out, so Nitrogen didn't help that, & wont help you either, if you keep your tires on for 10 years, the rubber has lost it's effectiveness anyway, so Nitrogen isn't going to help there.

I hope none of you would go without checking tire pressure for months at a time, and if you do, why bother with Nitrogen, when you obviously don't care about safety, mileage or tire wear?

People who buy Nitrogen, dont' even get me going on the environmental issue, change your oil & all your filters, check your tires, & keep your car tuned up, or stay quiet about the environmental impact, unless you ride a bike all the time.

The science behind Nitrogen sounds good to most people, but in practice, the first time you add air, (contrary to what the confused Steve S is talking about above???), you increase the ratio of O2 in the tire slightly, and eventually, the tire becomes the same mix as the outside air.

Go ahead, buy Nitrogen from the snake oil salesmen, while you're at it, buy a Q-Ray bracelet. Oh what's that? You already have one & it makes your life better? Brilliant. Spend the money on a tire gauge instead dammit & use it more often.

[Joe]

CR only bothered to look into one of the benefits of running nitrogen in tires. They did nothing to determine if tires run cooler when filled with nitrogen rather than air and they did not look into any issues regarding nitrogen being more "friendly" to the rubber. CR kind of dropped the ball on this one.

[tim hughes]

I tried an experiment using nitrogen in the rear tires of my corvette and air in the front. Although I could not tell any difference in grip, I have replaced the rear tires twice while the front are still in good shape. I agree that after using nitrogen on 2 different occations in the rear tires, they indeed wore out quicker with no difference in grip,braking or traction.

[Geri Rhodes]

Would it be possible to put C02 in tires as a way of sequestering it? What would be the effects on the operation of the vehicle?

[Mike Demers]

The missing piece of information? What was the purity or percentage of Nitrogen in the tire when all of these "tests" were performed? Like every other product, their are different levels of equipment and procedures for filling a tire with Nitrogen. The tire MUST have 95% or more Nitrogenn content. I see no discussion of the Nitrogen percentage in the above tests or personal expierences. Any reputable provider of Nitrogen will have a guage that can show the percentage of Nitrogen in the tire. The guage attaches to the valve stem of the tire, giving a real reading of Nitrogen purity. If your Nitrogen provider does not have a guage--you may be paying for compressed air! Independent blind tests by large fleet showed a significant increase in fuel mileage and tire life, when tires were properly inflated with Nitrogen. Not all Nitrogen services were created equal--95% purity or more is a must. If you check you tires every week, and fill with DRY compressed air--you will gain no bebefits. But, newer vehicles are going more miles between oil changes---sometimes 6 months and 7,500 miles. These long service intervals really demand Nitrogen in the tires. So a proper test needs...
1. 1 car with one front and one rear tire filled with Nitrogen.
2. tires moved side to side (some cars have different pressures front to rear, every 2,000 miles
3. Nitrogen content at least 95% in the tires
4. drive 8,000 to 10,000 miles
5. monitor the pressures at the beginning and end of the test
6. record the differences of pressures after 45 minutes of driving in hot weather
7. check tire wear at end of test
Will someone run a proper test? Or do we listen to more discussions with little facts or standard prodedures?

[Robert Treadway]

I think the CR test did exactly what is was intended to do: test the loss rate through the porosity of the rubber tires. The test was only intended to see if N leaked out faster than O2, under constant conditions. None of the temp fluctuations, driving conditions, "placebo effects", etc., can be included, because they cannot be held constant. The test showed a very small difference in pressure loss, which is all it tested for. You can extrapolate this to real world conditions: say you lose 3psi a month with air, then you can assume you would lose 2 psi a month with N.

The biggest (or only) benefit from N use, in my opinion, is the increased life of the tire casing, enabling it to be re-treaded 2 or three times, a cost factor in truck fleets. The theory is that the oxygen permeating the casing will eventually cause the steel plies to rust and fail. Joe consumer who replaces tires every 6 years will see no benefit. O2 in air and water vapor can rust the rim at the bead, causing leaks, but Joe consumer only keeps his car 6, or 10 years maybe?

[Fred Flintstone]

N sure sounds like another money making gimmick for the tire installers. A difference of only 1.3 psi air loss over a year's time is absolutely nothing. You should be checking your air pressure every 2 weeks anyway. Buy N for your tires and watch the mechanic laugh at how dumb you are behind your back.

[Steve S]

To Doug-

My comments about the percentage of N2 in a tire increasing was based on filling the tire, and topping it off, with normal 78% N2 air, as I stated. Yes, you're right that if the tire had been inflated with 95% or purer N2, then topping off with normal air would increase the percentage of O2 in the tire. However, that was not the scenario I described.

[Billy G]

Only a fool would believe that inflation with nitrogen would improve gas mileage 3-5 mpg. Well a fool and their money are soon departed.

[FRE]

One can, for little more than $100, buy an air compressor to keep in the garage. That makes it easy an convenient to keep tires properly inflated. With nitrogen, you have to go to a special out-of-the-way place during business hours to check the tires, so it will surely be done less often. That's why I have an air compressor to inflate my car, bicycle, and motorcycle tires. That also makes it possible to check the tires while they are cold.

[Don Lipman]

The atmosphere is already 78% nitrogen, on average, is it not? Therefore, by putting "pure nitrogen" into your tires, you're adding little additional nitrogen beyond what would otherwise be entered,

[Bill Babis]

Its just a money making gimmick. I suspect most of these increased mpg claims are due to a PSI change rather than the nitrogen. As far as 95% nitrogen not reacting with the tire rubber as much as 78% air, may be a valid claim but statistically meaningless. Unless you drive on a different planet than me, you have good old 78% nitrogen air reacting all over the surface area of the tire on the outside. Just check your tire pressures regularly and you'll save plenty of money not to mention whatever someone wants to rip you off for nitrogen.

[Bill]

The hypothetical MPG increase by using nitrogen in tires would be easily validated or refuted if Consumer Reports would fill the same tire once with air and once with nitrogen. In each case they could put the tire on their rolling resistance measuring machine. Then we'd know if there was a difference or not.

I don't see why there would be a difference but empirical evidence frequently surprises me.