rolling resistance article from tire rack tech

[GasSavers_SD26]

Originally Posted by DarbyWalters

High pressure are run in auto-x because tires don't have time to warm up and are doing lower speed sharp turns.

A lot of that is done in auto cross too because the sidewalls collapse, and the contact patch becomes the sidewall in extreme situations with street cars that might not have really low profile tires.

Kind of a good picture...

[yi5hedr3]

Interesting that the cheaper OEM tires give better fuel mileage than
the more expensive "better" tires!

[quadancer@bellsouth.net]

I don't see anyone considering that a narrow contact patch under higher pressures will wear out a tire much faster than normal, the load being distributed in more pounds per inch. This IMO, would offset some of your gas savings when you have to buy new tires much sooner.
I guess the question is just HOW many miles sooner over mpg over fuel cost vs. tire prices. I doubt it could be calculated off a track.

[theholycow]

Narrow tires should have worse rolling resistance, not better, per the link in my sig.

[ShadowWorks]

I think you need to drive each wheel onto a scale and calculate the weight each tire is loaded with, the do some maths and calculate the Psi for that tire?

I normally used to put my foot on the side wall and push the car, if the tire wobbled to much I would increase the Psi, but then some tires has soft side walls and some have stiff ones, its hard to know what is perfect for the least RR on your tire.

[theholycow]

Weighing each tire would let you fine-tune the pressure. There shouldn't be much difference across one axle, the left side should be pretty close to the right. The drivers side will be a little heavier because of the driver and the steering equipment, though I suspect the steering equipment is balanced by design with battery or other stuff in most vehicles. However, there are other reasons why pressure should be equal across an axle. I can't remember what they are though.

It would suffice to just weight each axle, which is significantly easier. In the US, and I'm sure everywhere, it's pretty easy to find truck scales for big trucks that need to weigh each axle; the scales are segmented for the purpose. The truck stop off I-95 RI exit 5B, for example, charges $7 IIRC.

[suspendedhatch]

You want the same tire pressure and same tires in the same condition across a drive-axle because of the differential. Having a difference on one side would be the equivalent of driving in circles all day long. If you have a front wheel drive car, then you can do whatever you want on the rear "dead" wheels. They have no steering connections and no differential. Rear wheel drive vehicles have steering on the front and a differential on the rear.

The weight on each corner of the vehicle varies more than you would think. Japanese vehicles are balanced somewhat, unfortunately they are optimized for their right-hand-drive layout. Yet I have serious doubts that varying PSI on each tire will give you any benefit to FE.

The proper way to set tire pressures is on a race track using a needle style pyrometer.

[GasSavers_BluEyes]

Actually, I believe Shadow is right. If you have a left-right weight differential, there should also be a different pressure in each tire to make the rolling radius of each the same. Of course, as soon as you add a passenger that would be fouled up, so we just set the same pressure in tires on the same axle.

I doubt any street cars outside of performance vehicles like Ferrari give too much attention to left-right weight balance. In a passenger vehicle there are far more pressing concerns than even a few hundred pounds of cross weight, things like fitting everything under the hood. Japanese are no better than any others. The transverse FWD layout is basically biased to whichever side the engine is on and different Japanese OEM's put the engine on different sides.

[theholycow]

I'm pretty suire that the radius of the tire is for all practical purposes exactly the same even with major inflation differences. However, if it was enough to matter, you'd want the same pressure left to right, because weight isn't going to change the radius, only pressure. I can't explain how I know that and I could be wrong, but I'm pretty sure about it.