Hypermiling – Evaluating common fuel-economy tips on the road

In "Hypermiling - The new menace on the road?" I wrote about drivers who compromise safety and traffic flow in an effort to get the most out of a gallon of fuel. And while some people may employ annoying or even dangerous hypermiling techniques such as driving 45 mph on the highway, turning the ignition key off when coasting, or tailgating a big rig for aero, many people are just looking for a safe and effective way to save fuel.

Some of the responses to my previous blog encouraged me to research and report more on hypermiling. I went to cleanmpg.com  for information from the "mild" to "advanced" ways to hypermile.

Armed with our Prius Touring that we tested for the June issue, I headed out on a weekend camping trip with my son in an attempt to beat the EPA cycle without doing anything unsafe or annoying to my fellow motorists.

After much study and practice, here are a few of the hypermiling tips that I tried, some successfully, some not so much:

1. Maximum tire pressure: Among the most-common hypermiling advice is to fill the tires to the maximum inflation pressure on the sidewall. However, this tip violates my safety criteria: You should inflate the tires to the recommended pressure set by the vehicle manufacturer. High inflation pressures can adversely effect emergency handling by upsetting the balance of the car, minimizing control on bumpy roads, and hurting grip on slick ones. It will also cost you money by prematurely wearing out your tires. So for a trip with my son, this one was out. (Learn more about tire safety and care.)

2. Keeping the speed down: OK, this is an obvious one. In our own fuel-economy testing, just going 10 mph slower can save a lot of gasoline.  Getting in the right lane on a desolate highway and holding the limit is one thing, but my trip was mostly smaller roads. While I tried to go the local speed limits, I quickly found myself being tailgated by annoyed locals wondering what was wrong with me. So I have to admit I did go a bit faster, but only when I was sharing the road with others.

3. Forced Auto Stop (FAS): The Prius Hybrid system allows the engine to turn off when not needed. It does so seamlessly and safely, and it is possible to drive in a way to maximize the engine’s off time. On a conventional car you would have to turn the ignition key off to make this happen (and then quickly back on so the steering wheel doesn’t lock up). But while driving in a car with the engine off may save fuel, the car may not save you if you need to stop quickly, accelerate, or turn to avoid something. Most cars will lose their power steering and brakes very quickly in such a situation, so don’t try this one. Because the Prius does this on its own, I did my best to keep that engine shut–off when I could.

4. Timing the lights: The trick here is to hit the greens as much as you can. Why go full-steam up to a red light, slam on the brakes, and then start off from a stop again when the green comes? Smooth out your driving by slowing before the red to keep some momentum as you go through the light. This takes some practice, but it does work. However, people driving behind you may not understand what you are doing and get frustrated as you slowly coast to the light. Another problem is it is easy to convince yourself that it is OK to coast through a stop sign or go through a light that has just turned red because you are doing your part to save the Earth. Do not do this! Actions like this lead to many deaths each year at intersections.

5. Pulse and glide (P&G): For me, this one was the coolest. For everyone who thought maintaining constant speed was the most fuel-efficient way – all things change when you are in a Hybrid. Under 41 mph the Prius can coast with the engine off without charging or using its batteries if you push down on the accelerator pedal ever so slightly. The P&G technique can only be used on slower roads, preferably with nobody else around you. The trick here is to accelerate briskly to 41 mph, and then coast as long as possible with the engine off and the batteries not being used, and when you get down to, say 34 mph or so "pulse" again back to 41 mph. On my trip, I took a lot of back roads and came back early in the morning. So with the road to myself, I wound up using this technique a lot. While unintuitive to do initially, when you think about the fact that the engine is only running several seconds each minute, the technique begins to make sense quickly!

Bottom line:
I drove 283 miles over the weekend and got (drum roll please)… 68.2 mpg average on the trip computer. Even I was surprised when I looked at the Prius-ometer at the end of the weekend. And 68+ mpg is downright impressive compared to the 42 mpg overall we got in our testing, but then again I didn’t do any of the stop-and-go city driving that we simulate in our tests. At $4.00 a gallon I saved about $10.00. But at what cost? I have to admit that in my concentration on fuel savings sacrifices were made. For instance, I missed a few scenic spots, talked to my son less, and at one point tried to talk him into "holding it" instead of stopping in order to maintain momentum.

When my wife heard about my mileage, she was eager to try out hypermiling. The next day we drove her 2006 Prius to the local supermarket with me at the wheel demonstrating my newfound talents. The trip was all back roads that allowed me to P&G nearly the entire way. The few lights I hit, I was able to time perfectly. As I backed into the space at the market (a hypermiling trick to allow a quick get-a-way, minimizing warm-up time) my wife’s jaw dropped at the display reading 92.7 mpg. On the way home, she tried it out herself. After about five minutes of driving, total fuel savings: None. It took us that long to make it out of the parking lot because she kept making wrong turns as she focused on the fuel economy display. She still tries to moderately hypermile her Prius, but she’s perfectly content getting 50 mpg and still having the ability to hold a conversation with our son.

—Jake Fisher

Also read, "Downsizing for vacation."

Comments

[David Traver Adolphus]

A lot of us in hilly areas do a Pulse & Glide lite. As you're doing the most efficient combustion under WOT, and you need to have the throttle open anyway, accelerate briskly up a hill and coast down the other side--engine on, in neutral. I realized I tended to stay in gear with the revs up in the past. This has yielded a safe 3-4 mpg improvement.

Oh, and stop driving stinking automatics. Why let that torque converter suck another 3 mpg away?

[Rob]

Thanks for taking the time to go beyond the sensationalist hype and take a more serious look at this topic. There are people who take hypermiling too far, but the vast majority are pretty sensible about it. I would also have to interject that most of the conditions under which hypermilers are deemed rude or dangerous has more to do with other people driving like idiots. If you try to drive the speed limit these days people will practically run you off the road. How dare you obey the law! If you try and drive efficiently between lights people get all annoyed and go racing around you, only to end up sitting at the next light right next to you. IMHO in general hypermilers aren't really the problem, they are the ones trying to be reasonable.

All of the tricks you mention above work almost as well on a normal car as they do on a hybrid. The Prius does have some attributes that make it particularly well suited, but that is not a requirement. P&G is helped in the Prius because the engine shuts off, but works in all cars due to the efficiency curve for a gasoline engine. The engine is least efficient at very light loads, like cruising steady at low speeds. By Pulsing, you push the engine into a more efficient operating region, and then by gliding use as little fuel as possible in between pulses. I would have to disagree with the previous poster though. At WOT (wide open throttle) efficiency drops off and emissions go way up as the engine control goes open loop. About 1/2 to 3/4 throttle is optimal in most vehicles to my understanding.

I live in an urban environment, so P&G isn't really an option. Still, by using the rest of the basic tricks outlined above I've taken my commute from 45mpg to 65mpg. Thats about a 30% reduction in consumption. If everyone were willing to make this simple sacrifice, we would have a substantial reduction in our foreign oil imports today rather than 10 years from now when CAFE standards and more efficient vehicles come into play.

[Kcissem]

your comment 1 about tire pressure is incorrect, higher pressures do not prematurely wear your tires, the opposite is true. Lower pressures equal better stability but higher wear(more tread hits the road), higher pressure equal less stability but lower wear(less tread hitting the road) as well as the added benifit of better FE.

[Gene Petersen - Consumer Reports]

Running a pressure below or above the recommended value suggested by the vehicle manufacturer will ultimately effect tire wear. Low pressure causes a tire to wear out quicker in the shoulders (tread at the nearest the sidewall area) and high pressure can cause the tire to wear quicker in the center. Ultimately a tire is considered worn out when any one groove across the tread wears down to the tread-wear indicator of 2/32". The correct pressure, the one specified by the vehicle manufacturer, is chosen for the most optimum and even wear across the width of the tire.

[Michael]

I've been trying some hypermiling for the last year or so in a car not normally associated with the task--an automatic 96 Subaru Legacy AWD. The gains have nonetheless been significant. I've been getting 27-29 mpg (measured per tank) in mixed driving and have routinely broken the 30 mpg mark with higher percentages of highway driving. Most estimates put the Legacy's mixed mileage at 22-24 mpg--so this is a gain of 15-30%.

Pulse and glide didn't give me any notable benefits (it might have in a manual) but I do a lot of coasting to stoplights and down hills (often in neutral), avoid going over 65 mph, maintain a space from the car in front of me and use the A/C and other accessories sparingly.

Of course, the best way to save on fuel is to drive less--so I've been trying to do that too.

[Chuck Thomas]

Thank you for taking the time to examine hypermiling and pubishing the results. It's refreshing to see a publication cover mainstream techniques and reveal that it's possible to gain 50% over typical driving these days....I was in an article this July in the Dallas Morning News driving slightly more aggressively from a hypermiling standpoint and registered a similar improvement with the reporter aboard, both legally and safely.

With the Prius, it's kind of like finding ways to make Windows or a Mac run better - they all have lots of software under the hood. Jeff - you might want to check with Dan at CleanMPG on how he uses ScanGage to do "Super Highway Mode" to coax the Prius computer to be leaner.

Hypermiling has a psychological aspect, esp in regards to tailgaters. 95% of the time punching the emergency flashers as a would-be tailgater closes in they just pass in a win-win situation. I do get a few people that are rude and about 80% of the time it's pickups, performance cars - obvious gas guzzlers/speeders....recently in the news some vandals have shifted from the Hummer to the Prius and these drivers likely have that mentality sadly. Bottom line for me is I've had not accidents in three years of hypermiling, but four the previous five years of aggressive driving.

[Chuck Thojmas]

P.S. I meant Jake when I said Jeff {blush}

[james]

A nice article, but I really suggest that you investigate your point #1. Automakers set their tire inflation pressures to give a "comfortable" ride - and I use quotes because their definition of comfort is almost invariably along the lines of Donald Westlake's "handles like a waterbed" squishiness.

If you check out any source devoted to performance driving, you'll find that higher pressures in fact provide better handling, without any extra wear. For one example, I drive mostly on winding mountain roads, and have run 50 psi in mine for years. The fronts still wear more at the edges than the center (just the opposite of what your theory would predict), while the rears hardly seem to wear at all.

[josh]

I can't believe people still argue over stinking tire pressure. The auto makers put that label on the door to optimize the ride quality of the car they are trying to sale to you. If someone thinks it is to optimize tire life and even out tire wear they are sadly mistaken. The correct pressure is the pressure the tire engineers have stamped on the sidewall. (This also should be done at the specified temperature which usually says cold, i.e 32 degrees farenheit.) That is the pressure that the tire carries its rated capacity and wears the most evenly. Underinflation of this number causes the shoulders to wear prematurely and overinflation beyond that number causes the center of the tire to wear.

[Doug Schaefer]

Re: CR's Gene Petersen's comment.

We completely agree that tire safety is of paramount importance, and that the *correct* tire pressure will lead to even wear across the tread (if there are no vehicle alignment errors).

We disagree that the vehicle inflation recommendation will achieve this in general. Large numbers of Prius drivers (posting on the internet) report that Toyota's recommendation (35 psi front, 33 rear) leads to premature tread wear on the edges. Inflation at or near the tires' sidewall cold maximum pressure (this will usually be in the 40's; check your tires) gives the most even tread wear for Prius.

Under-inflation is certainly the most unsafe because the tires will heat in highway driving and have greatly increased risk of failing on the road.

Everybody drive safely!

DAS

[Rich]

Read this:

http://www.officer.com/web/online/Editorial-and-Features/Driving-Under-Pressure/19$27281

"Driving Under Pressure Proper Tire Pressure Could Save Your Life"

I'm going to take SGT. DAVE STORTON's word for it and use the sidewall pressure in my tires.

If somebody tells me that it's unsafe, I'll need to see the proof.

Cheers,
Rich

[Bob]

It's sad to see the same tired complaints mentioned repeatedly, no matter how often they're debunked or denied. Hypermilers DO NOT recommend tailgating (I refuse to call it drafting because that adds legitimacy); they DO NOT recommend rolling through stop signs or red lights. The discuss coasting in neutral both with the engine on and off, but warn about loss of power assist when you do and recommend it only under EXTREMELY LIMITED conditions. Personally, I never use either. Oh, and BTW, ignition off does NOT lock up steering; In general, auto transmission vehicles need to be in PARK, and the LOCK position is another click or two past OFF.
Regarding tire pressure I agree with Doug Schafer's post: the sidewall markings are best for wear and MPG. The auto mfg specs are more for comfort than performance.

[Comparison Shop Event Tickets]

Re: all the comments about tire inflation. The original poster is wrong in the assumption that manufactures always specify the optimal pressure. And it seems like the PC people are wrong to assume that near the maximum is the best (aside from chasing pure FE). The best way to tell if a tire is optimally inflated is to take a piece of sidewalk chalk and draw across the tire tread. Drive a bit. If there are any marks of chalk left the tires aren't optimally inflated (I believe if there are some in the center the tires are under inflated, and if there are chalk marks on the outside the tires are over inflated).

[donee]

Hi CR,

I agree with Doug Shaefer. The safest tire pressure is the pressure where the tread wears evenly. Because this is the pressure where the contact force between the road and patch is constant all the way across.

One of the findings of the Explorer rollovers investigation was the recomeneded tire pressure was too low. So, when the tires ran down the highway in desert temperatures, they overheated. This was a MANUFACTURER's Recommendation that appears to have been the partial cause of many deaths, and injuries.

Michelin has a video of what happens in hydroplaning conditions when the center of the road contact patch is not a proper pessure. Filmed up through a clear piece of glass , the tire seperates from the road surface, and a thin film of water forms between the tire treads and the roadway. Only a small sliver of road contact remains on the edges of the tires. A higher air pressure tire maintains tread contact with the road surface all the way across the width of the tire.

If the tire pressure is so small in the center of the patch, the tire will not wear as much there as on the outer edges. This is the condition found on the Prius with the Toyota recommended tire pressures. 42 PSI on the front tires, and 40 PSI on the rear tires is what most people at Prius Chat recomend for this car.

Other cars may be different. Use careful measurement of tire wear to determine the safest tire pressure.

[Jim Kerr]

Great tips.

I add only that you could also try to slow down sooner as you approach a stop sign or light.

My big question with the Prius is "does shifting into low gear while approaching a stop feed more energy to the battery than staying in the drive?"

jk

[donee]

Hi jk,

The Prius transmission is automatic in its operation, but does not have a 2 or 1 position. The B position is there for downhill engine braking. Decellerating in B charges the battery less than the P position, as allot of the braking would be from the engine drag. Going down a mountain, or very long hill, the battery can become too charged to accept any more electricity. So, they provided the B as a backup.

In normal operation, in P , to get electric braking, one simply lifts the gas peddle. If more is wanted, press on the brake slightly.

Electric components heat up when used, and all metals increase resistance with higher temperature. So pushing electricity through the higher resistance disipates electricity before it gets to the battery.

The efficiency of generation is poor at low generator speeds, as the generator voltage does not exceed the battery voltage as much. The Prius actually uses a generator with a nominal voltage twice that of the battery to increas the fraction of the braking where electricity can flow backwards. The harder one brakes the speedier the generator turns, and the higher the regenerated voltage for the braking period. You can hear that inside and outside of a braking Prius.

So , there is an optimum braking rate to recover the most energy. That is light braking does not charge as efficiently, and heavy braking heats up the components and wastes some electricity as heat. Extreme heavy braking will trigger the friction brakes, too.

For best mileage in normal traffic, its best not to stop if you do not have too, in the Prius or any car. Because while the Prius does recover some energy, the recovery and reuse is not 100 % efficient, combined.

[donee]

Hi Bob,

There is no assumptiong going on by the Prius Chat people. Remember the second generation Prius has been around since October of 2003. Its nearly 5 years on now. Many many sets of tires have gone through many many cars and been commented on at Prius Chat.

The people getting flat long life tire wear are those running 42/40 PSI.

[BobM]

When I first attended airline pilot initial ground school (almost 40 yrs ago) it was stressed that dynamic hydroplaning begins at a speed of 9 times the square root of the tire pressure. Using these numbers, a tire inflated to 35psi would begin hydroplaning at just over 53 mph, while one at 50psi would be good to almost 64 mph. Difference in auto vs aircraft tread patterns (cross-grooving for cars) & road surface variation (grooved highway) may skew these numbers, but you get the idea.

[unclesharkey]

If you just drive like a normal person you are going to increase your mileage. I took my 96 Honda Odyssey from MD to RI. I drove it at a moderate speed between 60-65 mph with 3 passengers and a full load of luggage in the back. I averaged over 28 mpg. I did noting special just kept a steady speed and did not drive like an ass. No need to do all this silly hyperspastic crap.

[Steve O]

Good story. Thanks.

What's your terrain like for 60+ mpgs? My Prius rarely does much better than 56-58 mpg in an area full of small rolling hills.

In town, on level roads, with light traffic (allowing very for very gradual braking and slow acceleration at traffic signals), I've been able to hit 80 mpg. But never in "2 acre" subdivided suburbia where I do much of my driving.

[Gary]

I began the Prius adventure in 2004. As many above stated. Point #1 ?? You took the knee jerk reaction about tire pressure, as you originally did with the entire hypermile topic. Find a designer for one of the major tire brands (already been done by some on these blogs) and you'll be surprised. You'll find that as to tire pressure, one more re-write is in order. Don't worry, you're getting there. BTW, another technique for driving hyper is (if you can choose your own) commute time. I could work from 7 to 4, but choose to work so that the commute begins at 4am. So I'm homw before 2:30pm. Very light traffic at both ends, so the 'other-guy' isn't inconvenienced.

[OEM Ride And Handling Engineer]

Regarding the tire pressure comments, I can tell you that the OEMs absolutely set the tire pressure considering ride comfort, fuel economy, wear, AND handling. I can say that because I'm one of the engineers that assess the handling performance of those vehicles at that tire pressure. The tire pressure stamped on the sidewall is the maximum pressure for the maximum static load on the tire, which is normally more than the vehicle is rated to carry anyway. If you choose to exceed the recommended tire pressure for some small economy improvements (and the returns are diminishing because I've seen the data), you are changing the shape of the tire patch and the response characteristics of the vehicle. Even if I grant you the assertion that the tire pressure is arbitrarily set for comfort (which I don't), then it is still a risky practice because all of the setup for the vehicle (handling, steering, ABS, stability control) are done, and therefore optimized, at that pressure. If you want to up the tire pressure on your vehicle that is certainly your option, but don't kid yourself or others about the potential consequences.

[used car dealer raleigh]

the post is correct tires wear out at higher pressures. that's what we have seen so far

[david]

To OEM Ride And Handling Engineer and CR:

Some questions/comments here:

Why don't manufacturers give choices to consumers as to what they are trying to achieve? For example, for my sports car, I want the best possible handling air pressure. For my daily drive/sedan, I want the best combination of "ride comfort, fuel economy, wear, AND handling."

Do each different brands/models of tires have a different optical performance characteristics for a particular vehicle?

What are your thoughts on using the probe type pyrometer? Useful? Useless? Why bother with such an expensive device?

Appreciate your thoughts.

david

[JoeG]

Regarding tire pressure, actually with the Prius, you get a more uniform treadwear with higher than Toyota-recommended tire inflation pressure. Check www.priuschat.com This phenomenon is not true with most vehicles, therefore, your assumption about the Prius is incorrect.