Tire pressure and fuel economy RESULTS
 

I searched through the many many tire threads for this and while I learned more about tires that I ever thought possible I didn't find anything quantifying results of different air pressures (underinflation, overinflation, and hyperinflation). MetroMPG, I read that you were planning to test and post results--did you ever do it?

A little context: I bought low RR tires for my truck and seemed to increase FE on average about 8%, but overinflating (even to just 35 psi) only seems to help the truck ride rougher than it already does. I haven't had the cash to buy a scanguage yet and am determining my FE old school (like the manual recommends): miles driven divided by gallons used. While a good system for an average FE, it isn't so good for showing the effects of small mods like this.

So I'm wondering how much tire pressures are helping anyone else because if the effect is negligable I'm going to put it back to help keep my coffee off my lap!

My tires are at max (50 psi) and I can tell when they are down a month or so later when I have to reinflate. My car coasts in neutral approx. 1/3 less with 35psi vs. 50 psi. There are times when I'm at max psi that I don't touch the gas pedal for close to a mile on my commute to work.

While I don't have stats to back this up the only two mods I made starting from 17 mpg before ecomodder was max psi and driving technique. When my tires are at 35psi my mpg is down 1-2%. I know it works...no doubt. I'm 33% over epa and I noticed 1-2 mpg gain after inflating my tires to 50 psi.

If you check my fuel log and look at the 27 mpg I pulled on a summer roadtrip I guarantee it is all due to P&G with 50spi. I used to get 22 mpg on those trips a year earlier.

Is it what you are looking for ?
http://ecomodder.com/forum/showthrea...ures-2721.html

From placard of 33 psi, I inflated to 43 and felt a change in coasting. When inflated to 46 I felt even more difference and in city it seamed to coast forever... I inflated to sidewall of 51 yesterday evening.

http://ecomodder.com/forum/showthrea...auge-4369.html
Personally at each fill-up I calculate the saving in money, and I save that amount for investment in my cars. I already bought some anti-rain and anti-fog products, some new hubcaps, a voltmeter and a little solar panel (for the battery I had to change but that isn't totally dead, so I keep it to feed my (GPS) navigator).

As numbers from MetroMPG post are significantly different, you can guess that with the proper driving techniques (DWB and coasting) you'll get success. Changing habits do need work, but success will be significant.

Denis.

Heh! I'll back you up, mang... :thumbup:

And, you can take this from the World's Record Holder for Honda B16A2 FE!

50 psi is THE sweet spot for regular street tires... not 36 psi or even 44 psi... I've tried them all - door sticker to 50+ psi!

If you're a real tweak and run 'Land Speed Record Tires' - Bonny Tires - you can get away 60-70 psi, but I think that's getting a little extreme...

Anyway, pump them babies up to 50 psi and I guarantee you'll see that everything Matt Herring said above is 100% true! ;)

Um...

I suppose it goes without saying - but we're talking auto tires here, not truck tires, which require considerably more psi...

Is this with your tires rated to 50 psi? Mine are rated at 44, which is where I keep them. I know some are fine with overinflating, but I'm not as of yet.

In MetroMPG's experiment, 44 psi rated tires are used and they get their better score at 50 psi.

Of course different brands, different results.

Denis.

That result could be well within the margin of error for the kind of testing that was done.

We know that rolling resistance as a function of inflation pressure varies more or less linearly. A tire with more inflation pressure will always have less, or at worst the same, rolling resistance the same tire with less inflation pressure.

How much it impacts FE depends on the vehicule, mainly what portion rolling resistance represents compared to other sources of drag, but you can safely assume that more pressure will result in the same of better fuel economy.

This past Saturday morning I slugged down to the Toyota dealership to get the 'ole oil change. They nicely greet you at the door...process your car and then whisk you away to a comfy waiting area with 42" plasma TV's on the wall (full breakfast bar available if you want).

During the processing of my vehicle the "attendant" that does your paperwork (who is really a salesman) asked if I wanted the tires rotated. I obliged and mentioned that the tires are at 50spi and I'd appreciate if when they rotated them that they kept it at 50 psi (every time before they dropped it down to 35 psi without asking). When I mentioned the psi the attendant asked why I had them so high and here's the conversation (roughly what was said)...

Me: "at higher psi there is less rolling resistance which gives me better mpg's while I'm coasting in neutral."

Him:(after a chuckle), "You believe in that stuff, huh?"

Me: "I've driven 1500 miles over epa for free since June partly because of my higher psi. Haven't you seen more and more SUV's available on your lot and less cars available since gas prices went up?"

Him: (in a robotic tone) "We can barely keep the SUV's on the lot and higher psi ruins your shocks and wears your tires out faster."

After this comment I expected the phone call an hour later to upsell new tires and shocks to me...thankfully it was only $25 for new wipers.

So with no interest from him in talking about saving gas I pedaled my feet away. And, as a fact (because I counted while I was waiting for my vehicle), they have exactly 3 used and 4 new prius cars available on the lot vs. 26 4runners, 42 tacomas and 28 tundras (and 22 FJ Cruisers). When I bought my 4runner in May of 2007 it was the only 4runner left on the lot!

Maybe its because i'm cheap, maybe its because i'm an engineer, maybe its because i'm honest... but i loath salesmen, and that's a funny story. I just hate that he says the same thing to everyone else and 9/10 believe him. I'm sure he'd tell you it was a lucky day that there were >100 guzzlers on the lot... total stroke of luck.

Tyre pressure is directly related to the weight of the vehicle. 50 PSI in an SUV is safer than 50 PSI in a small hatch back. If you over inflate tyres you will have a smaller contact patch on the road hense the lower rolling resistance. This can create uneven wear, generally in the centre of the tyre, as this is the area with most contact with the road.

I do over inflate by a few PSI, but not much, because I don't want to compromise safety for a 1 or 2 extra mpg.

I'm nor a layer, nor a tire expert.

Over-inflating between placard and sidewall is in safety limits given by constructor and manufacturer.

Over-inflating over sidewall is certainly dangerous under unexperienced people's hands, but can be safely used by experienced people :
Driving Under Pressure (full article) - CleanMPG Forums

Denis.

my tires are at 50psi and are rated 51psi. from 35 psi i went to 44 and picked up a little over 1 mpg. i went to 50 psi and saw another 1 mpg. in a 17 city epa 4runner that drives 90 percent city thats a huge gain. i know im getting at least 2 mpg more with 50 psi.

my in car mpg gauge read 16 to 17 mpg before eccmodder and i got it up to 20 mpg before i started logging my mileage on ecomodder. if i drove more highway miles i guarantee you id be pulling 25 mpg in this car but with only a 4 mile commute to work on all 35 mph flat roads in city driving im happy with a 5 mpg gain since february.

with 90 percent highway driving on a road trip in august i pulled over 27 mpg in a 22 mpg epa highway rated car with no mods other than psi and p&g and no instrumentation (scanguage is coming for christmas).

pump those tires up boys!

Interesting thread.

I think hyperinflated tires (my definition: tires being inflated past the mfr's maximum) are a big mistake. Sometimes people get myopic about fuel economy and forget that you have to steer, stop and corner. A hyperinflated, or overinflated for the load, tire reduces the tire footprint and changes the whole roadholding dynamic and the way the tires, brakes and suspension work together. In the long run the money you might be saving will just come out of your paycheck in another way, such as in a set of tires much sooner. Or chiropractic visits ( : < ). Or a sudden, devastating tire failure. MetroMPGs second test should help convince anyone that there's a plateau for any tire.

I experimented with my newer pickup and discovered, as MetroMPG did in his latest test, there's a "Three Bears" just right and any gains above that point are fractional. In my case, the truck manufacturer's loaded pressures of 50 psi were it. I didn't see any marked gain by going up another 10 psi to the tire sidewall maximum. It also turns out that the difference between the tire placard max and a slightly lower pressure that offered reasonable ride quality, performance and tire wear was very small. When I upgraded to larger, more farm field friendly tires (taller and slightly wider), I expected to lose a little mpg but I actually gained some, most likely from the "overdrive" effect of taller tires (truck has 4.10 towing gears) that cancelled out the losses due to rolling resistance and gave me a little extra. Surprisingly, the height gain didn't throw the aero off too much, at least at the moderate speeds I cruise at but I did lose noticeably at 70 mph and above. I might be able to get some of that back with a bed cover. Will be doing some coast-down tests next spring to check it all out.

heres my contribution

i have mud tires on my daily driver. yeah, i know.... i'm working on it.


anyways, they have a 50 psi max rating. i've been running them at 45 psi with good results. i've noticed my last 2 tanks were about 2.5 mpg lower than normal. so i checked my tire pressure, and they were all at around 33 psi. guess i need to check them more. i know the colder weather made the tires "lose" air, and i'm sure the colder temps are affecting mpg due to longer warmups. but i pumped them back up and waiting until next fill-up to see if it made a difference.

Economic sense of high tire pressure
 

Hey folks,

All this stuff with hyper inflated tires is fine but does it make economic sense ?

Overinflated tires will not last as long as normally inflated ones so while you save gas, you loose tire life ! Time for a reality check maybe :rolleyes:

If you really want to save by more rigid tires, get a solid rubber wheel or better yet, a TWHEEL !

Michelin Tweel: en vidéo par Le blog auto: actualités automobile et sport automobile

Cheers

Denis
Lion sur Mer
France

Salut Denis,

This is sounding more and more like a myth. Every fuel economy enthusiast I know of who overinflated have seen increased tire life and even wear. There are a lot of people overinflating and reporting successful results. Yet, we don't hear about those who overinflated and got excessive center wear. Maybe because that's just not happening.

tasdrouelle: Well, yours is not exactly a scientific observation, so I'll offer one that's of equal value scientifically: I've been in the car business professionally since 1970, that's nearly 40 years now, and I have seen tire wear, belt separation and suspension problems due to overinflation many times. And I've seen tires that blew out due to it. Seen them first hand. It's happening!

Tires are a lot more complex than they appear. If you want an brainful, read the SAE book, "Tire Forensic Investigation" by Thomas R. Giapponi. You'll see the multitude of ways tires can fail... in gruesome detail. Call it CSI for tire guys ( : < )!

Yes, people get away with overinflation. That's partly because most tires are over-engineered. Plus, if you have to go one way or the other, overinflation is better in all areas than underinflation. So if a guy goes from checking his tires pressure every three years (whether they need it or not) and generally wearing out tires due being underinflated, then "getting religion" and going to the opposite extreme is going to seem like a whiz-bang concept that yield great results. If you compare tread life of a properly inflated tire vs an overinflated tire, you'll see some more unhappy numbers.

The over inflation saving graces for this Ecomodder group are that, a) they generally drive small, light cars, and b) those who know about such things buy tires with a high durometer and a tread design that offers low rolling resistance. These factors all contribute to a lower wear potential than the average tire (lower performance too, but...). Also, without wishing to insult anyone, logic dictates that a guy who destroys his tires by hyperinflation is not all that likely to advertise it in an, "I'm a bonehead," kinda post.

Me, I'm more concerned with the braking and handling issues. A little experimentation via a skidpad and stopping tests will show the loss of performance that comes from overinflated tires. The threshold is different for every tire/car combo and there may be a point where fuel economy and performance meet. Based on my knowledge, I'm willing to bet that point comes well under the tire's maximum pressure at any given load.

My little practical test showed the fuel economy difference between overinflated and properly inflated was too small to measure on my truck. Metro's rolling resistance test hints at much the same thing on his little car. To me, that makes the risk avoidance part of this very easy. If I discovered there was a 3 mpg gain in overinflating my tires, I'd crunch the numbers, do a little risk vs reward assessment and maybe give it a try. But, alas, all I got at 60 psi was a molar-loosening ride and squirrelly handling.

What Goodyear taught me
 

I was considered a tire expert in a former life. I got tours and info that others did not. Here are a few bits for you...

1) DOT tires are tested to caryy 15% more pressure and weight than is listed on the sidewall.

2) Heat is a huge enemy of tire wear. Over inflated tires run cooler than underinflated tires.

3) The first 32nds of tread wear occur more quicly than the last usable 32nds.
Think of a pencil eraser.

4)For best tire mileage, combining 1,2&3 Put new tires on the car in the late fall and max inflate them. The cooler temps and extra air will help the tire wear more slowly when it is most vulnerable, thus giving extended tire life.

5)Plastic valve caps are only dust covers and the shrader valve is only supposed to be a temporary seal. Use a metal cap with a seal for higher pressures. For best results, use a metal valve stem with a sealing cap.

My response....
Prove it :rolleyes:

I've got 20-25K miles on my tires inflated to 55 front, 50 rear.... Wear has been even, I still have more than half of my tread left.

Plus, I live in an area that gets a lot of heavy rain - having less hydroplaning risk has been a nice benefit :)

If you don't want to believe me (and all the others on this board with the same experience)... How about this?

And for thoroughness.... Stopping distance


FEA chapter III. tire pressure survey and test results

And that ends this reality check :thumbup:

You people seriously need to stop quoting studies that compare Proper to Lower. 17 psi test results are completely irrelevent when discussing 35 vs 55.

I have raised my tires from 36/33 to 45/42. I notice longer coasts, rougher ride and DECREASED cornering and stopping ability on dry pavement. I race my car (on race tracks, not strips or ovals), so i can communicate with my tires well. I won't lower my pressures due to this decrease in grip, but that's only because I am a HIGHLY attentive driver compared to the masses, and I have high speed vehicle control training. If you think you know what you are doing, remember that 80% of drivers feel that they are better than 80% of drivers!

If you drive like a granny ALL the time and you stay well back from people in adverse conditions no matter how late you are, feel free to bump them up over 50. All others are in blissful ignorance land.

^^ The data shows no reason for there to be a knee in the curve. It's shows there is no correlation between contact patch and stopping distance - we could have just believed what we learned in our physics classes, but it's nice to have an excuse to play with our toys {err, cars}.

The data also shows ABSOLUTELY no reason to believe that there is NOT a "knee in the curve". The experience of every person that I've known that races is that there is a sweet spot on the pressure curve. A change of 1psi from that sweet spot can result in a second or more lost on a one-minute track. Lap Time is inversely proportional to grip. PERIOD.

Grip is proportional to static Mu (mechanical system properties of rubber/road) and mass... Period... (<--or three of them :p)

Therefore

Lap time is inversely proportional to Mu and mass....
^I'm not seeing contact patch here.

Unless you're talking about drag racing... In which tires are designed to rely heavily on adhesion under conditions road vehicles/tires won't see...

Ed,

Where'd you get that one? That is just not true!!

Treb,i don't get your last post... changing from the 'sweet spot" psi brings you away fromyour best lap times. Therefore there is a drop-off in grip above the sweet spot. Simple as that. Throwing in a bunch of physics terms doesn't make you right when you are using them wrong. Go to a performance school. Ask if you should gradually adjust your tire pressures to find the sweet spot, of if you should pump them up to 100psi for max grip.

When the conditions of driving on the road match the conditions of driving on the track, I'll be doing that... This isn't the case.

As for using physics incorrectly - you need to correct me if you're going to claim it to be wrong ;) Additionally, I am not throwing anything around in the sense that it's unnecessary. I only brought up applicable information. If you're not going to correct me while stating it's being used wrong, you're simply throwing it away ;)

The conditions on the track EXACTLY match those on the road when you need to use the maximum tire grip on the road, such as when you need to avoid running over a child. 4 wheels on an asphalt surface being asked to provide as much cornering or stopping force as possible.

Let me clarify. You differentiated Mu from contact patch. However, you did not realize that contact patch is only a side effect of inflation, but the relavent side effect of inflation is mechanical grip, which IS in your loose equation. So yes, Mu*mass is a loose representation of your grip, however Mu is very much dependant on tire pressures, though not necessarily contact patch.

Sorry, but tires do not conform to the classical physics interpretation of friction: F= uM (gee, I wish I could post Greek letters!!)

That's because the tread rubber penetrates the road surface texture. That results in a situation where the max grip is acheived at a percentage slip - which classical physics says is the regime of sliding friction (and at a value lower than static friction).

See figures 6.7 and 6.8

Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire

OK, I give up ..too much magic not enough engineering
 

Guys, I quit Ecomodders !

People who tell you that WAY overinflated tires dont wear abnormally are dreamers, liars or drive like grandma. Wanna really saves ga ? Remove the brakes from your car -théy're HEAVY:rolleyes: - the handbrake will do...

NO, I quit THIS list, serious, evolving discussion does not seem possible here. It's like those HHO lists where people claim to ignore thesecond law of thermodynamics abd make a perpetual motion machine...cause they wanna !

I only wished that serious people here would start another list with engineering and safety minded ecomodder discussions.

Adieu !

Denis
Lion sur Mer
france

But denis, way underinflated tires don't work as well as perfectly inflated tires. So way overinflated tires must be WAY better!

I'm going to chug 9 mugs of coffee, i'll be back in 5 minutes. I'll get SO much work done!

As I'm fairly new to this group, I didn't check to see if this topic had been dealt with before. I just looked. Yeeeha! Y'all been down this road before back in May and June! I shoulda known! I see some of the same handles... and arguments. Some interesting data has come out, now and back then. Good stuff to file away. I doubt we'll reach a consensus here (one could say there's no such thing as a "consensus" on physics and science anyway... it's either correct or not ), but it's sure a fun road to ride down... whether your tires are overinflated or not!

Yes, this one is quite the can of worms. All i have is a few years of experience in racing and tuning tire pressures for heresay, and all "they" have is data saying that underinflation is no good. Believe neither of us till one of us comes up with a repeatable, properly conducted test. I'm hoping to have a G-force meter next year. I'll use it IN MY STREET VEHICLE to prove one of us wrong.

PLEASE don't go Denis! It's the nature of the Forum beast. You often have to reach through some "brown trout" to find the diamonds. Sometimes not fun...but they're diamonds and the brown stuff washes off. Second, if you leave, you abandon us to exactly who you describe. Plus, you are the the second "Denis" I've met (as opposed to a "Dennis")! we need your reasoned voice.

Argg... I had a rather lengthy post written up to revise that post to bring in more variables... Mainly, Stiction (one of my favorite engineering words) and mechanical locking - but Solidworks crashed on me and brought down my entire machine :/ If I can re-find the road tire tribology article on ebsco, I'll come back and link it....

In light of that frustration, and Denis' post - I'm going to break my normal operation and abandon ship... My apologies, I'd be equally pissed on the flip side of that coin....

Before the door hits me in the ass

Personally, I don't see how to directly link driving like grandma and tire inflation :p I personally don't drive like grandma - unless legally abiding by a speed limit classifies as that (feel free to gloat about breaking the law, however :D). Sorry to see you go, however :/

Nobody tell him how many engineers are already on this board :p

Lastly,
Not way better - as originally posted

Well, I have a G-Force tester. Never used it for braking tests but it has that capability. If you and CapriRacer want to outline some test parameters, I might be able to find some time to try it before winter hits. Pretty busy right now, but I can try. You see what rolling stock I have available. The Honda is the obvious choice, though it's tires are on the thin side (jeez, those PROPERLY INFLATED Michelins are wearing out after only 70,000 miles). The newer Ford has fresh meats, but they are 285/70R-17s. The old Ford has fairly new meats too, but it has sucky brakes (sucky from the factory, I mean).

I also have some tire company contacts and I'll see if there are any other studies.

Wow, what a landslide of info! I'm the original poster.

Sorry that you are leaving Denis, but I have to say that this roll down tire lane has been of real benefit, including especially the link to the post on rolldown tests by MetroMPG.

The result for me: I'm keeping my tires at 35 (max inflation on the side of the tire). The tire size I'm using was rated for low RR by Consumer Reports, but sadly the size of my specific tire has 35 max pressure on the sidewall unlike most others of the same model which take 44 psi.

I am no tire expert but I worked at an alignment shop for years and saw many tires with the center of the tread worn out from overinflation. And many many tires separating. Often those were underinflated (which as described above can cause tires to overheat). But too much pressure (as stated somewhere above or on another discussion) can cause fatigue. Rubber doesn't stick to metal well and I for one don't want to "hyperinflate", go over a manufacturers max inflation, and potentially cause separation and an unsafe situation. The max PSI are what the manufacturers have tested the tires to take, and even if they can take a little more, I'd like to have that margin remain as a margin of safety.

So thanks to all the help I am going to keep filling with nitrogen at max inflation and checking the pressure regularly to keep it right at max inflation pressure.

Thanks for the tip on metal valve caps. And for all the fruitful discussion! :thumbup:

Jim,

Don't bother. The testing that will prove this is going to trash the tires under braking. The only other way would be to do skid pad testing of lateral G's - but that's going to trash the tires, too!

Not to mention that heat in the tread compound is going to affect the test results, so the only way to get a good test is to have many tires already mounted up and a guy standing by to do the changeover.

Interesting. Are you using R-compound tires? Those generally grip better at relatively lower pressures. Hard-compound "real street" tires usually grip better at higher pressures. I do know that the tires I used to run (Yoko AVSes, that'll tell you how long ago it was! ;) ) gave me better lap times at 40 PSI than they did at 36 PSI.

The above was autoXing, which places a significant premium on lateral grip. Braking and acceleration are not as important, so you can still get better lap times with worse longitudinal traction as long as the lateral grip is better.

Other tests I have seen recently (a recent Grassroots Motorsports issue) showed that higher pressures on the less-grippy end of a car (I think it was on the rear of a CRX) will result in more grip to a point--then the grip falls off. The higher pressures generally resulted in better feel (AKA "harsher ride") and less loss of grip than being an equivalent amount of pressure under the "maximum grip" pressure. However, the grip was less consistent, so it was sometimes harder to control. (I hope I don't have that last statement backwards, it's been a month or two since I read the article.)

I talked to some drag racing friends of mine, and they seem to agree that lower pressures let you launch better, so it sounds like for acceleration a lower pressure is better. I haven't seen much of a consensus for braking distances, though.

...Oh, and I have the tires in my CRX just below the sidewall maximum.

-soD

Well, to throw another wrench into the discussion - there isn't really enough data in those tests to make a conclusive decision. Not enough runs done.

Bummer! Yeah, that makes sense. Well, anything that trashes my tires is a no-go fer sure. I'll fall back on my tire contacts and see if I can get some test info on the tires they trashed. That study that was linked to from Goodyear was interesting but it really didn't answer the question of if, or whether, braking performance degraded at the high end of the pressure range, above the 35 psi listed (which I took to be the max pressure rated for the tire). They were looking at the low end pressure... which, truth be told, is the scary end they should be looking at from the safety standpoint related to neglectful car owners.

I think it was you that talked about a "sweet spot." I'll bet there's one that offers the best of all worlds, performance, safety AND fuel economy, and I'll bet it different for every tire/car combination.