Nitrogen in tires - Q&A

Our recent blog post, "Tires - Nitrogen Air Loss Study", looked at using nitrogen in tires. So far, it has generated a lot of interest. In fact, that post has received more comments than almost any other. Among the comments were many questions--more than could be addressed in a simple comments format.

Senior tire engineer and program leader Gene Petersen has compiled answers to the comments thus far, adding further insights into the testing and the results. A tires Q&A is presented below.

For further discussion, online subscribers are invited to engage the experts and other consumers in the "Tire Talk" forum.

For quick background on the nitrogen test:
Consumer Reports wanted to find out if nitrogen is worth the price for passenger vehicles, so 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.

Read the original post.

Q: Were the tires installed on a vehicle? Would the weight of a vehicle have a greater effect on the loss of psi?
A: The tires were initially inflated indoors at room temperature and then stored outdoors for one year on a specially built tire stand to keep them separated. After one year the tires were returned to the indoor location and inflation was checked at room temperature. Our test was a controlled comparison with many tire models. Plus,  we did not install them on vehicles, which may or may not have been another variable.

Q: 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.
A: That's  a logical conclusion.

Q: Were the tires on new rims or older ones?
A: The tires were taken from a tread wear test we performed last year. The tires had 16,000 miles of simulated travel on the wheels, so technically the tires and wheels are used products. But I want to mention that the tread wear test ensured that the tires were well seated to the wheels.

Q: 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.
A: Fuel economy is related to the tire's rolling resistance, which is a function of load and inflation pressure. High load or low pressure causes a tire to have higher rolling resistance and, therefore, lower fuel economy. If the nitrogen retains the pressure better than air in a tire, fuel economy might benefit. But I cannot think of another reason why fuel economy would solely improve based on using nitrogen in lieu of air. Overall, if you can maintain proper inflation pressure, then fuel economy will be optimum.

Q: 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.
A: Yes, nitrogen makes up most of the air -- about 78% as you point out. Think about this, though: if you fill your tires with air, the oxygen is more likely to permeate out of the tires before the nitrogen and over time you end up with a higher concentration of nitrogen. I have not checked this but it seems possible.
An additional point: not all air is of equal quality. Moisture, oil from the air compressor, and other pollutants can affect tire integrity.

Q: I just had nitrogen added in our 1999 [Saab] 9-3. I'm getting roughly one mpg better than before.
A: Interesting. Maybe the tires were under-inflated before you had the nitrogen fill.

Q: 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?
A: The intent of the study was simply to answer how well air and nitrogen are retained in a tire. This methodology allowed us to chart the differences across more than 30 tire models in controlled conditions -- a laboratory process we would not have been able to replicate driving the vehicles. We did not explore the claimed benefits of nitrogen on limiting tire aging or look at the dynamic effect of gas permeation under operation. Those interesting topics were beyond the scope of this test. Also, wear effects would largely be affected by maintaining proper inflation pressure regardless of the gas used (air vs. nitrogen).

Q: 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 inert 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 over-inflation and wear/tear resulting from heat build-up.
A: Interesting points. Because nitrogen, in our case, is a processed gas (moisture and oil was filtered out by our nitrogen generator), you might expect better inflation control as the tire heats up under normal service vs. air with unregulated moisture, etc. And nitrogen has been shown by the government and industry to reduce tire aging.

Q: This seems like a flawed test because the tires weren't tested under "normal operating conditions."
A: Tires are designed to perform as intended with air, and the tire manufacturers tell us as much. Assuming that proper inflation pressure is maintained,  the tires will run as designed using air or nitrogen.

Q: What about daily, weekly, and/or monthly fluctuations due to significant changes in the ambient air temperatures? Would you expect that the psi variation due to ambient air temperature changes over shorter periods of time could be much more pronounced?
A: We did look at the inflation pressure over various ambient temperatures but could not find a significant difference between air and nitrogen. We are not making any claims here, but just telling you what we found. 

Q: 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 100 psi.
A: The positive benefits of nitrogen in high(er) service pressure applications, such as used in large truck tires, has been documented in the industry. Our test centered on passenger tires, only. We are not discrediting the use of nitrogen, but it is not a substitute for regular inflation checks.

Q: I think the bottom-line comment about being sure to check pressure regardless of air or nitrogen is key. I think the tendency would be to NOT check nitro filled tires as frequently as air filled.
A: I agree. People might think that once nitrogen is used to inflate tires that they no longer need to check the inflation pressure. Hopefully these tire blogs and discussion with our faithful readers will be a good reminder that there's no substitute for regularly checking your car's tires.

Learn more about tires and see the results from our latest tests in the Tires section of ConsumerReports.org.

Comments

[Eperry]

Most people dont check their air pressure - true. Nitrogen takes longer to permeate or leak out - true. Nitrogen generators filter/dry/clean/ the gas fill into the tire - true. Nitrogen is inert - True. Oxygen is an oxidizer - true. Water vapor will assist the oxidation/corrosion process - true. Water vapor will cause greater fluctuation in pressure over temperature change - true.
After reading all this and the studies my time is worth getting nitrogen in the tires. If you make $5 an hour skip the nitrogen.If I get just a fraction of an MPG from my car at 3.50 a gallon or a few thousand miles of tread life or any reduction in tire stress aging I would be comfortable with it. Consumers should simply report the numbers, all the numbers, and the true numbers so they can maintain their reputation. Their readers are typically above average - this is really not a big deal to understand.

[Charles]

A lot of people seem to think that not having the weight of a car on the tires in the Consumer Reports would make a difference. It would make no difference at all. 30 PSI means that the air or N2 in the tire is pressing with a force of 30 pounds on each square inch of tire and wheel. This is true no matter what weight is pressing down on the tires. So if you fill tires on a car to 30 PSI the tires air/N2 is trying to escape at the same pressure as if you filled the tires with the car jacked up.

[Jerry]

Even with all that has ben written above, and if we assumed that dry nitrogen is better than pump air, what advantage do you get when the tire installer slops a whole bunch of water-based "lubricant" to enable the tire mount. A whole bunch of this water is in the tire as is the air that naturally filled the unmounted tire. So the fill with dry nitrogen is adulterated with tire mounting liquid and the air in the unmounted tire. Does any shop purge all the water introduced? Do they purge the air? I doubt it. Besides, the nitrogen used to fill tires is 95% nitrogen as opposed to air which is 78% nitrogen -- not that much practical difference. Water and water vapor is the real culprit and that is not managed in real tire shops.

[Bill]

I recently purchased new tires for my wife's car and searched all the comments on nitrogen. Many claimed that their tires retained air better with nitrogen. As an engineer I looked closely at all comments and came to the following conclusions - 1. Little improvement would be achieved by increasing the nitrogen content from 80& to 90% 2. If the tire place has impact wrenches, they have a filter dryer, so humidty is not a problem 3. The only noticeable difference is the valve cap with a seal in it. The valve stem has always been the weak link in tires. Plug that leak - with or without nitrogen - and your tires do not leak. I have the sealing valve stem caps and my tires consistantly hold their pressure.

[Gordon]

Interesting comments on 'saving' the tire's rubber by using Nitrogen... now if we could just 'surround' the outside of the tire and prevent that pesky oxygen from touching it, we could make our tire (rubber anyway) last forever!!!!
Sheesh... fill your tires with the correct pressure for the time of year, check it weekly and drive the darn things... new tires don't last much longer then 8 years anyway!!!!

[Duane]

Re: Nitrogen. I have a car with aluminum rims and had a constant problem with low pressure. I changed to Nitrogen, the problem is gone. I live in Canada so temperatures change dramatically. I purchased new tires and seller offered Nitrogen at no cost. I am sold on Nitrogen.

[Michelle]

I had nitrogen put in my tires a few months ago. I have a 2004 Chevy Trailblazer. Last week I got 361 miles on one tank of gas. I have had this SUV for 4 years. I could never get more than a little of 300. You do the math.

[Joanne]

This whole controversy over nitrogen in tires is silly. All gases (air and nitrogen) obey the ideal gas law, PV=nRT, in which P is the pressure, V is the volume (in this case of the tire, and we may assume it is fixed), n is the number of moles of gas in the tires (just a quantity of gas), R is the ideal gas constant, which varies from gas to gas, but is still a constant, and T, the absolute temperature. Since V, n, and R are constants no matter what gas we use, it is clear that when the temperature rises, the pressure of a gas (in a fixed volume) must rise. When the temperature drops, the pressure drops. This holds just as true for nitrogen as it does for air. I will admit that there is probably less degradation of the inside of the tire due to oxidation if the tire is filled with Nitrogen, but the outside is being oxidized anyway. The whole idea of nitrogen diffusing out of the tire less readily than air is crazy: the oxygen molecule is somewhat larger than the nitrogen molecule so oxygen would diffuse out of the tire less readily than nitrogen. However all of this diffusion business is on so small a scale as to be essentially meaningless. Bottom line: check your tire pressure monthly and forget about this gimmicky nitrogen thing that flies in the face of all rules of physics and chemistry.

[Terry]

I read with interest all the posts and faqs above and found it quite a source of entertainment. Wow, you folks sure are serious about this Nitrogen vs. Oxygen and I certainly learned more that I intended. All the mathematics and physics and scientific discussions I admit are beyond any reasoning I care to study at length.
I am more concerned with the structure, safety, material and quality of the tires I consider purchasing than what is inside them. I bought a set of tires a few years back that were so bad that I figure I may as well have filled them with money!
Nitro may well be better than that plain 'ol wet nasty air I have been using for years. Then again it just might not be. I don't know but I will let those of you more intelligent than me fight this one out. I reckon this issue will be an issue for sometime to come with little decisive resolution.
In the meantime and since my mind is thinking about it I think I will go out right after this post and check my tire pressure....well, I'll just check it on one side because if they are close to being correct then I'll assume the other side is ok too. Next month I'll check the passenger side.
You know there have been Snake Oil Salesmen around for years and years and as of right now it seems to me this Nitro business is one and the same.
Now where is that old physics book? LOL!

[lulu]

i use nitrogen in my truck , becouse is free at cotsco , i would never pay for this otherwise, mpg is the same and even if i save some & in gas, nitrogen cost about $40 and tires last only 6 years, they expire after 6 years , i like the idea is safe and leak less that's ok

[Walt French]

"Tires are designed to perform as intended with air, and the tire manufacturers tell us as much."

Yes, I'm sure. It'd be pretty unlikely for Ford, Toyota or whoever to say, "these tires may explode and kill you unless you use a particular approach that only 1% (tops) of all cars employ. Not much incentive for them to say that their products are defective by design for their intended use.

That said, it seems unlikely that the N2 solution is worth anywhere near the time & energy expended. This is an engineering question and anecdotal "uncle in Dubuque" stories of 10% better mileage seem pretty lame... tells you more about the writer's beliefs about what to believe when there's not enough time to collect more hard knowledge.

[Donna Myer]

I appreciate all the opinions and data and science shared here.

After discussing the nitrogen vs air issue with a group of physicists, chemists, and environmental chemists - my conclusion had already been that the issue was WATER - as a vapor or liquid depending upon the temperature.

SO I contacted the service manager to tell him why we had decided not to use his expensive (that is actually an understatement) nitrogen.

AND HERE IS WHAT HE TOLD ME- that when they put air into the tires of the new vehicles on their lot, EVERY MORNING the "tire pressure monitors" on the dash in the new cars tell them that the pressure is TOO LOW. But when they put in nitrogen, the monitors did not indicate low pressure in the morning.

Now THIS is a NEW take on this issue - for if "the automatic, tell you when your tire pressure is too low, sensors" on the new cars tell people that the pressure is low with air and not with nitrogen -- that kinda looks like PROOF. But of course it is NOT proof of what they say it is proof of -

I was told "nitrogen simply does not change pressure when the temperature changes." WRONG.

I was told "oxygen leaks out of the tire faster because it has smaller molecules than the nitrogen." IRRELEVENT.

I was told "oxygen in the air has water with it - your know, H two O! Nitrogen has no water with it." OH, good grief!

I can see the following taking place - someone richer than me buys a new car WITH "tire pressure sensors" reporting on the dash and without nitrogen in the tires.

The first AM with low enough temps over night, and with the tires having set and cooled down, that sensor will tell the driver as soon as they start their car that their tires have too low a pressure and the driver needs to add air! Since only my husband (the physicist) uses a bicycle pump to raise his tire pressure by 2 or 3 PSU -- the unhappy driver will drive until a station is found where there is an air pump. AND since low pressure tires flex more and flex causes temperature in the tire to rise, as does the rolling friction of driving - when the driver gets to the air pump - the sensor may no longer say that the tires need to have added air! SO the drive drives to work and if the car sits for long enough for the tires to cool down AND the ambient temperature is below water's dew point - the sensor will give the same "not enough pressure" info when the driver arrives to drive home. This only needs to take place a couple of days for the new car owner take the car back to the dealer reporting a faulty sensor!

With the DRY nitrogen in the tires, this water vapor to liquid will not happen, the owner will not be continuously reminded that the tires need more air AND the car dealer will not have to handle a bunch of unhappy buyers who seem to have defective sensors. Given the lack of understanding of the way gases and water vapor react to temperature changes, and the lack of understanding that tire pressures will rise when tires heat up, the science ignorant service personnel will give out more false science explanations as reasons to use their (expensive) nitrogen services (as the dealer told me - these machines are not cheap you know!)

YET another reason the dealerships are pushing for nitrogen! AND the BAD SCIENCE knowledge of the general public goes on and on and on -

SOMEONE should come out with a box with a tire stem on one side and a little air hose on the other. In the box should be that blue hydrophilic rock you can buy and hang in your basement - and a little battery powered heater ... Hook the air pump to the valve stem side of the box, connect the box's air hose to the tire and .. VOILA You could then get DRY AIR everytime you got air. The blue crystals can be reactivated in a heated oven to give UP their adsorbed (yes that is the right word) water - and reused. We could sell the little box for HALF of the fill up cost for four tires - and get rich quick! AND NO NEED TO DRAIN OUT ALL THAT OLD NITROGEN AND REPLACE IT WITH FRESH NEW NITROGEN!
I certainly hope that all that nitrogen is organic - eveyrone know that organic is better!

CHEERS - a frustrated science teacher

[Jay]

Donna (the science teacher). Re your "little box": my post above of Apr 24, the bottom paragraph, they DO make just what you describe. Some models of the "dry air pump" have just the sort of blue silica crystals of which you speak!! (about 1 1/2 quart) When they start to get pink, you put them in a Microwave for a few minutes -- they get dark blue again, and you put them back in the air pump.

AND the sensors on car tires are soon going to be standsrd. My 2004 Explorer had them standard ... I kept the tires at 33psi, and had a nail in one tire, so a slow leak. The alarm on that tire came in at 25psi. (repeatedly) A couple of months later, I had a leaky valve core on another tire. THAT one alarmed at more like 27. Good to have!

anyway, give Teacher an A!

[Vera]

Just yesterday, I had nitrogen put in my tires. They are fairly new, being installed on my car in February, 2008. While the service person did a good job of answering my questions at the time, now that I have thought about it overnight, more questions have surfaced: What happens if I drive over a nail and have a flat. Can I drive replace that one with the spare [that does not have nitrogen - at least bill says 4 tires] with the other 3 nitrogen filled tires? If so, how far?

None of the questions/statements shown above answered that specific question. Help.

[Winston Elston]

I have a compact pick-up usually driven daily and a land-barge class sedan that often sits for months without being moved. I have alloy rims on both, with Michelin tires appropriate to the vehicles. I have never observed any loss of tire pressure in the Roadsmasher despite the infrequent use. Recently the Frontier also sat idle for nine weeks, and a check of tire pressure before remobilizing it indicated no loss of pressure.
I keep the tires of both vehicles at the pressure specified by the vehicle manufacturer (30 psi for both), and I assume that the tire sales facility at Sam's Club uses air.
This is not a scientific study in controlled and monitored conditions as many of those commenting have posted, but a real-world, typical conditions anecdotal observation. I conclude that as long as the seals between the tire and rim and the rim and the valve stem is good, and the tires are in good condition, nitrogen offers no advantage.
If the vehicle is maintained at a dealership or other professional maintenance shop, the tire pressure is checked and adjusted at every service interval (for my truck, that is usually monthly, for the Buick, twice a year) pressure loss should not be a problem as long as the tires and rims are in good shape.
Temperature changes will cause pressure changes no matter what the gas or gas solution is used, as confirmed by the ideal gas law.
I see no advantage to using nitrogen, and if your tire installer uses it "at no charge", you can probably find the same tires for less elsewhere.

[DIANE STEWART]

do I need to buy winter tires when my tires are filled with nitrogen
and do they perform the same in the winter as regular filled tiresa

[Tim]

Francis:

"Oxygen also destroys the characteristics of rubber over longer periods, known as dry rot."

Thats fine... how do you keep the oxygen from attacking the OUTSIDE of the tire? I've never had nor heard of a passenger tire in normal use failing due to oxygen deterioration.... I've heard of UV damage.

[David_In_Austin]

Thanks, Jay, for all the great info.

I'll buy the argument that it's the lack of moisture/oil rather than the lack of oxygen. However, quantitatively, what is the magnitude of difference that moisture makes on tire pressure versus a dry-air filled tire? Presumably the effect is more apparent at lower temperatures. Are we talking 0.5 psi difference, or 3 psi difference, or 5+ psi difference?

Obviously, 0.5psi isn't worth caring about. Your tire gauge probably isn't that accurate anyway. If it's just 3psi, why not just overinflate your tires by 3 psi? Most tires can take it without complaint, and then you're even with nitrogen in cold weather. If you have to choose, you're better off overfilled than underfilled. On the other hand, a difference of 5psi+ might be worth the hassle of using dry air, because then you're getting a meaningful margin of safety for your effort.

Also, if the moisture freezes at the bottom of the tire, then I guess it could throw your tire out of balance (until it warms up in operation, anyway). Still, how big an impact is it?

[gordon rieger]

I'm thinking of using nitrogen in my tires, since I have a new car with a tire pressure sensing device, will the sensor still accurately measure the tire pressure?

[Mr.Wizard]

Jay is exactly right on. His explanation is as good as I could find on any discussion boards. There is one additional safety reason for N2 in large aircraft mandated by NTSB: fire (N2 does not accelerate a fire like compressed air).

[DaveK]

Jay is right. Nitrogen effuses (leaks from small holes) faster than oxygen, not slower. This is called Graham's law, and it says that gasses leak in inverse proportion to the square root of their molecular weight. Oxygen (o2) weighs 32 grams, and nitrogen weighs 28g. Nitrogen leaks 7% faster than oxygen. Consumers report will have to come up with another explanation for the observed pressure losses. The difference is so small, I suspect that it's not statistically significant.

[Guelphite]

I wholeheartedly agree with Jay.
1. For average drivers the difference between nitrogen and air is simply too small to matter.
2. Garages who buy the expensive nitrogen filling equipment are being told things by equipment salesmen that are simply over the top - but once a garage has bought the stuff they have to pay for it.
3. I agree with those who are skeptical about mileage gains with nitrogen - once you get interested in this issue, your behavior changes.
4. My own experience: our garage talked my wife into taking nitrogen into the tires. I live in an area where there are marked changes in temperature and I measure pressure frequently. The pressure fluctuated significantly (yes, Boyle's Law) but the kicker is: do I go to the garage every time I need to adjust the pressure ?
5. If you put in air to make up the pressure while traveling do you have to go and bleed everything out of the tire when you return before you refill with nitrogen ? The whole thing is one monumental sales triumph, against all common sense.
6. And, by the way, the average car in the US and Canada does not travel at 40000 feet and does not go at speeds in excess of 200 mph, and the average service station does not let you top up with nitrogen.

[john stropparo]

Nitro machines remove virtually all moisture, winter temps won't cause ice in the tires. Nitro molecules are larger so they don't leak through smaller size imperfections in the rubber. If the efficacy of the nitro is intact the impurities will be lower. Less rust from impure metals from your average air compressor.

[Michael Pelletier]

It would be interesting to compare a nitrogen fill with a very dry air fill, such as from a scuba tank - scuba air must be very dry, especially in cold water diving, to avoid having moisture freeze during the endothermic pressure drops in the regulators delivering air to the mouthpiece, as well as to prevent corrosion of the tank's interior.

I wonder how much of the difference is simply humidity, rather than molecular dynamics.

[Arnold Dyck]

I bought four new tires in Oct./08 and the dealer filled the tires with nitrogen. This is their standard practice and charged $5.00 per tire!! It seemed to make some differnce as far asleakage was concened, but only minamal.
My concern is that as the temprature drops to -20 C. and lower, the tires a very noisy on cracks on the highway!! This characteristic was not mention at all.
I find the comment by Jay, he shows that Nitrogen does not have larger molecules that oxygen!! very interesting. Here goes the main selling point.