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

[skewbe]

sigh, we can go through this all again, or we can re-read the previous debates:

http://www.gassavers.org/showthread.php?t=1727#6

People here are reporting even tire wear with lots of pressure and 10s of thousands of miles. Many of the overinflated tires here are pretty skinny to begin with and being steel belted radials they seem to hold their shape better with extra pressure.

You have steel belts going in every direction, if it were to bulge out in just one part of the middle tread it would have to pull in from another part of the middle tread, but that part of the tread also has extra pressure forcing it into the shape defined by the steel belts.

I know it seems counter-intuitive and unconventional, maybe the center bulge theorists are a hold over from before steel belted radials? I don't have enough miles to consider myself a first-hand expert, just trying to think it through in the mean time and make sense out of the reports of "overinflating works".

[omgwtfbyobbq]

IME it really depends on the tire. My passenger car tires have shown no wear after thousands of miles, but my light truck tires definitely showed the lower tread in the middle and alla that.

[garyhgaryh]

Here's a reason not to run too much pressure, granted I filled my tires to 40psi and the tires are rated to 35psi.

http://www.gassavers.org/showthread....&highlight=age
Gary

[skewbe]

Originally Posted by garyhgaryh

Here's a reason not to run too much pressure...

Not exactly, it happens to "normally" inflated tires too. Given that kickflipjr doesn't know the history of the tire in question, it may also be that an abused tire (pot holes, underinflation, manufacturing defect) is less suitable for over inflation, but it would also be less suitable for driving on in general, and one incidence does not a study make.

I love how lug nut fixed his with rubber cement His failed at 32 psi.

[trebuchet03]

Originally Posted by skewbe

Not exactly, it happens to "normally" inflated tires too. Given that kickflipjr doesn't know the history of the tire in question, it may also be that an abused tire (pot holes, underinflation, manufacturing defect) is less suitable for over inflation, but it would also be less suitable for driving on in general, and one incidence does not a study make.

I love how lug nut fixed his with rubber cement His failed at 32 psi.

He did say

It was probably the combination of old cheap tires and me hitting a bump or pothole hard.

And we keep saying, don't over inflate old tires For that matter - don't even use old tires....

[omgwtfbyobbq]

Originally Posted by skewbe

Not exactly, it happens to "normally" inflated tires too.

X2. I hate an entire set of Contis fail. But I think it had more to do w/ my mom leaving 'em out for days on asphalt during summer since I never overinflated those. For that matter, I had decade+ old tires fail after I overinflated 'em 5psi... But I don't think the fault was because of overinflation as opposed to them being about half my age.

[jandree22]

yeah I'll concede that when searching around after I posted what I did above, I see that playing with sidewall max isn't a prescription for disaster as I previously assumed. Particularly with Prius owners, those that used OEM specs had their outer edges wear out super fast because of increase heat/flexing, and those that used sidewall max had minimal wear, even including the middle of the tire.

And yeah, brucepick, your points about load capacity vs. psi make perfect sense

[skewbe]

Wow, 60 huh? Braver than me Weight distribution could be a factor in the difference.

[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

[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.