"Inflation pressure does not affect grip": Autospeed article.

[orange4boy]

My toyota pickup (1989) called for 35psi, this would make the rear of the vehicle harsh, and unpredictable.

I assume you meant when your pickup bed was not loaded.

We are mostly talking about passenger cars here but I agree that pressure should be proportional to load as this makes sense. I run lower pressure in my rear tires than my front as my loading is not 50/50 front / rear.

I am not espousing over inflation in this thread. Extreme over inflation and low loading may cause the centre to wear faster but max sidewall and proper loading does not.

I am not saying there is no relationship between inflation pressure and traction but that it is not as drastic as many people believe and running higher pressures, within reason and up to max sidewall, is safe and has more benefits than downsides.
Added: Tire pressure should be proportional to load, ie: a car with 55% front 45% weight distribution should have slightly lower pressure in the rear tires. One suggestion is to use the same ratio as the placard.

Benefits of max sidewall pressure in my opinion from what I have learned:

1) Lower heat build up. Reduced risk of accident or rollover resulting from loss of control from heat related tire failure.
2) Reduced wear
3) Slightly increased peak traction, slightly reduced sliding traction. Slightly reduced stopping distances (may have to do with increased traction)
4) Reduction in hydroplaning (onset of hydroplaning happens at a higher speed)
5) Reduces slipping in snow* (depending on conditions, sometimes lower pressure is better)
6) Increases tire responsiveness in cornering.
7) Reduces body lean in cornering, improves steering feedback (subjective)
8) Reduces fuel consumption
9) Reduces steering effort

Downsides to max sidewall:

1) Slightly harsher ride quality.
2) May increase rollover tendency in already rollover prone SUVs in certain conditions.
3) Harder on suspension components in rough road conditions
4) Slightly increases risk of tire bruising or failure from impact with blunt objects.

I have been updating this list as I learn more just so you know I am open to the most credible information.

[vtec-e]

Honestly, this topic could go on forever. The main reason i have lost interest in it is because there has been zero proof (from what i can see) of how traction is affected when your tire is at sidewall. Mine have been at 50psi (44 max) for the past year and i have noticed and measured no uneven wear. I have however noticed the wheels bouncing over bumps while braking and the resulting loss of grip. So i think i'll drop off the pressure a little so my shocks can keep up with the bumps in the road.
I've always ranted a bit on this subject but i feel an about face is in order.
In the lab, traction may not suffer at higher pressures but my roads are far from lab conditions and my shocks can't keep up with them. 44psi it is from now on then!

ollie

[gone-ot]

...divide the total weight of your vehicle by four, that's the rated load the tires you're using should be rated at, or slightly above.

...since rated load does increase with air pressure, you can tweek things up to the "max" sidewall airpressure.

...I'm assuming this increase in load rating is linear, but it's probably not actually linear.

[Christ]

I run Gerald (1/2 ton F150) with 50-60PSI (I'm lazy, so it's never accurately one number) in his LT235/75R15 tires... I notice a harsher ride, but hell, HE'S A TRUCK. I didn't buy a truck because I was a ninny that wanted to ride like I was in a family sedan, I bought a truck to be able to haul, make money, and have the ability to tow more than just a pup trailer. I don't notice braking issues on Gerald, either, even when bouncing over bumps... why not, you ask? Well, for one thing, I hardly ever use my brakes. For another thing, even if I were using them as much as everyone else, on my truck, the fronts do the majority of the braking anyway. I would lose little or nearly none of my braking power (unloaded) by losing traction in the rear of the truck while braking, unless I'm using engine-assisted braking, in which case the engine would stall.

[Frank Lee]

I've worn out the centers from overinflation. Others say they haven't. Believe it, it is more than theoretically possible. YMMV.

I don't care about traction- as rmay says, know your car and you can handle it.

I won't run more than 50 psi anymore simply due to higher pressures shaking the **** outta me and my vehicle. The theoretically fractional fe gain definitely isn't worth that.

[dcb]

Darins testing indicated a sharply diminished efficiency return @about 45 psi, on his metro (referred to in post 94). In lieu of your own testing , I wouldn't bother with going over that. Would love to see more testing however.

[Christ]

My sidewall max is 50 or 55... depends on which tires I have on.

Anything less than 45, there is noticeable sag in the sidewall.

[dcb]

well, you do typically have a larger vehicle than a metro, which is why some testing would be useful. IIRC the 18-wheelers put 100+ psi in their tires.

[Christ]

I ran 85 PSI in my 11.00x20's, and 135 PSI in the 14x22.5's in the jockey trucks. Suspension seats kick ass.

Once I finally get off my duff and hook up the MPGuino, and the truck is actually street legal (inspection), I'll be more than happy to do some instant economy tests and report back.

[dcb]

I have to say I really liked Darins methodology, he eliminated a whole lot of variables by letting a hill do all the accelerating. Of course translating that to mpg gain is tricky, but it isolates the psi effects on efficiency pretty well.