How much of an increase in mpg can I expect with overinflated tires?

[Ernie Rogers]

Originally Posted by theclencher

patch size is largely a function of weight on tire over tire psi

Hmmm, that's right--- the load on the tire is equal to the force at the pavement, which is pressure times area: F = P A

wider tire, all else equal, will have same size patch area BUT different shape- wider and narrower front-to-back.

Yes, the area will always be the same. The question is, how does the flexing of the rubber compare? The strain in the rubber is generally proportional to the energy loss. [Energy loss = (1-k)(strain energy) where k is the coefficient of restitution.]

ernie: skeptical about dbl psi/half rr claim. also don't agree with not exceeding max sidewall listed psi

Think it through and you will conclude that the amount of strain is equal to the patch area, which is inversly proportional to the tire pressure.

Okay, we can disagree on exceeding side wall "max pressure." I prefer to believe that the engineers that set "max pressure" really meant it.

on point 2 ya really hafta get nuts on oversizing tires before it throws the speedo off by more than a few mph

I want to know who told you I was crazy-- but it is about getting good mileage, not tire size. :-)

on pt 3 i posed this question some time ago and the answer was lo pro tires roll EASIER.

The question again is which tire has less total strain in the rubber. A tall sidewall accomplishes the needed deflection (to make the flat patch) with much less distortion needed. I will grant that the low sidewall can be made thinner, which helps in the other direction. But, most people have reported lower fuel economy with low profile (= low aspect ratio) tires.

on pt 4 if you return new tires cuz fe goes down you may never end up with new tires!!! cuz worn tires have lower rr. it's tough for new ones to beat that

Aahahaha, that's correct. You will have to use some judgement here.

Ernie Rogers

[Ernie Rogers]

Originally Posted by lca13

>>ernie: skeptical about dbl psi/half rr claim.

Inversely proportional makes sense, but not linearly and not with a 1x1 slope, if nothing more than because drive train resistance is not affected by tire pressure. Max mpg gain is probably when you start really slipping in the corners, and max mpg loss is probably when intitial resistance is "much" larger than actual rolling resistance. In either case, your tires may not last very long :-)

The inversely proportional rule is the theoretical, rolling straight down the road. The theory is never exactly right in practice. The really fussy competitors have a much more complicated formula for rolling resistance, not a constant Crr. See for example "The Leading Edge --Aerodynamic Design of Ultra-streamlined Land Vehicles" by Goro Tami

Ernie Rogers

[Ernie Rogers]

Originally Posted by Minger

I'm still stumped as to what I should set mine as, both the manual and tires say a max of 35...maybe I'll try 40 next time I fill up?

I use 40 psi (44 sidewall) because it's usually cool when I set the tire pressure and I don't know how hot it might be later-- unless I'm in a competition.

[omgwtfbyobbq]

Originally Posted by Ernie Rogers

Okay, we can disagree on exceeding side wall "max pressure." I prefer to believe that the engineers that set "max pressure" really meant it.

I think it's safe to say that the max pressure also corresponds to the max load, at the max speed, on a pot-hole ridden road, during a Death Valley summer day, plus some safety margin. By figuring out what the pressure is during those conditions, and comparing it to the conditions we tend to ride in, such as moderate weather, a max speed of ~65-70mph, less than max load, I think we can take a decent stab at how much cold overinflation is still safe for ourselves.

[brucepick]

Originally Posted by jandree22

I'm a member of the don't-exceed-sidewall-spec group. The owners manual is a suggestion based on estimated load in the car. There's leeway built in for people that load their car full of other people and misc. heavy cargo. That's why it's safe to screw around with that number. ... OM = 34psi; SW = 44psi; Mine=40psi

wadr [with all due respect],

I have to say, for my own car I've settled at a very similar practice to your decision. My oem is 27 with optional increase to 32 for better FE, my sidewall 44, my tires are at 40ish.

Please don't guess that loading up your car/truck will stress highly pressurized tires. It's the other way around. Increasing tire pressure raises a tire's load capacity, up to the tire's sidewall rating. So, at least according to the published information, running at less than max sidewall gets you less than rated capacity and increasing beyond that point gets you no additional gain. You can find a lot of this at tirerack.com if you dig for it.

I'm putting in another plug for the reference quoted earlier in this thread:
http://www.officer.com/article/artic...on=19&id=27281

Long story short, they put Crown Vic police cruiser tires (44 psi rated) + wheels on a Ford Ranger p/u and pumped them up to 100 psi. Then gave it to a race driver to do some stunts with. Success. The tires hold up far better at 100 psi than they do at 44.

There is quite a lot of additional information in the article on the topics of safety, handling and reliability of tires at different pressures. Draw your own conclusions, but please do read the article.

I'm not recommending running 44 psi tires at 100 psi for street/highway use. However I'd certainly consider the max sidewall to be allowable, and 10-20% more to be doable. In actual practice, 40 psi feels pretty hard in my car so I'm not inclined to go much higher. But that's my car and my tush. As usual, ymmv and I suppose ytmv also.

[trebuchet03]

The sidewall is the max pressure that the specific tire manufacturer determined is safe. Basically the point where they shed liability if the tire blows up while you're inflating it up to 55psi.

The offs of a blowout while inflating are quite low (unless you've got a old crappy tire).... It's more of a blowout when you hit a pothole. In any case, the max pressure is based off of a minimum factor of safety.

So I did a quick search... and found a patent for bias ply tires... It makes a reference that says the steel cord safety factor should range between 4 and 11, 7 being a target. Now that's not directly related to inflation pressure - but should be related to hoop stress (P*D/2).... Again, this isn't really an equivalent - and I don't know if the mentioned factor of safety applies to traditional radially belted tires... Really, I doubt I'll find the actual number anywhere as it's probably trade secret.

----

Yes, the area will always be the same. The question is, how does the flexing of the rubber compare? The strain in the rubber is generally proportional to the energy loss. [Energy loss = (1-k)(strain energy) where k is the coefficient of restitution.]

So that makes sense and all...

A tall sidewall accomplishes the needed deflection (to make the flat patch) with much less distortion needed.

However, (just thinking intuitively now) - shouldn't we be comparing the deflection delta? That is, a shorter (and stiffer) sidewall doesn't deflect as far when conforming to the road (stiff/bumpy ride) as compared to a larger sidewall with a lower k value...

I am also thinking of a case of a tire with wheel run out (out of round). So we basically have a tire in the shape of an ellipse (a very minute one though). The areas with the shorter sidewall will have more stiffness and translate that into the the vehicle suspension - as compared to the higher sidewall. So that, combined with the assumption that the normal force is constant leads me to the difference in taller versus shorter sidewall is the k. Thus the deflection for the stiffer (shorter sidewall) should be less than the taller sidewall...

So that paragraph above would mean that sidewall stiffness AND inflation pressure are variables that determine contact patch size (something tells me stiffness from the tire is much less than inflation stiffness)

Of course, this could be *** backwards - and x is constant and the force changes (although, intuitively - that seems wrong or I am missing a key point).





In any case, it doesn't matter for me either way. I have no intention to get a shorter sidewall I just like this sort of discussion

[brucepick]

Originally Posted by Ernie Rogers

I use 40 psi (44 sidewall) because it's usually cool when I set the tire pressure and I don't know how hot it might be later-- unless I'm in a competition.

My research on tire pressure has always found statements to the effect that
1) max sidewall is spec'd for cold pressure, meaning cool morning temps before driving the car.
and
2) max sidewall and also the mfg oe recommedations allow for increases in outside air temp during the day as well as increase in tire temperature from driving.

So, check tires when cold. Drive with confidence after that.

[garyhgaryh]

I wouldn't run your tires way too much over what the side wall states.
Over inflating your tires will wear out the center. If you think about it, it will also give you less contact patch to the ground, that is one reason why you are getting lower rolling resistance and getting better mileage. But you pay for it at the end with premature tire wear. Something to keep in mind. You get short term benefits of better gas mileage at the expense of your tires.
Gary

[skewbe]

Overinflating is not really a cause of premature wear, low pressure is the major cause of that.

It may affect wet handling adversely, so practice. Dry handling effects are good, the sidewalls are nice and stiff and do not flex as much in the corners (them guys that drive on two wheels use like 100 psi).

Them steel belts keep things lined up pretty well so there's no appreciable bulging from overinflating.

[DarbyWalters]

Low Profile Tires on a larger diameter rim for the same or bigger tire diameter won't hurt mileage ( I know you meant on the same sized rim but it could cause confusion).

There is a point where rolling resistance will increase with tire pressure...if the tire is set so high it stops conforming with the road surface, it can be detrimental.

Tires do grow convex and/or convex with different psi's. This will happen in the middle of a tire and causes different running temps along the inside, middle and outside edges of a tire...you really want pretty even tire temps for best results. No reason to get uneven wear on tires and then all the money you saved on fuel goes to new tires. Agreed that low psi's are much worse, so err on the side of high psi's.