RES pretty much answered your Qs from one perspective, so I'll go down the
less traveled hotrodder's path......
Take the max weight that your tire can support and divide the 'max cold' psi
(pounds per square inch) into it.
This will give you the max weight your tire can support per square inch.
Example... my RX7's tires have a max weight of 1356 lbs. @ 44 psi cold, or
1356/44 = 30.818 lbs/psi.
Then, take your true axle weight at front and back. Divide by the number of
tires on each axle.
Mine is approx 1750 lbs. up front and 1980 lbs. in the rear. So, for the
front, 1750/2 tires= 875 lbs. per tire, rear = 990 lbs.
Then divide that number by the max weight per square inch figure derived
from step 1.
875/30.818 = ~28 psi per front tire, rear = ~32 psi.
This number gives you the required psi for supporting that weight and the
absolute minimum the tire should ever see.
Since my tires can hold 44 psi, I am safe to run anything up to that
pressure. Mazda recommends 32 psi F/R, though I currently run 36 psi F/R to
offset the effects of the mix of Yokohama Avid (front) and BFG Comp T/A
(rear) and inside to outside wear between F/R due to turn-in scrub on the
front and negative camber scrub on the rear.
Then there's the shade tree engineer's way......
A simple way to determine if your tires are inflated properly is the Chalk
Test. Draw a thick line straight across the treads. Now drive straight so
that the tire goes around a few revolutions. Get out and inspect the chalk
wear. If it's worn off in the center but not the sides, then you have too
much air, if the opposite, then you have too little air.
Note that a tire's pressure changes by ~1 psi for every 10 degF of ambient
temperature change, so at the least, changing with the seasons is a good
idea for vehicles driven year round. Also, tires generally lose 1 psi/mo.,
so for best all around performance, check them regularly when cold.
GM
----- Original Message -----
> what tire pressure do you run on radial tires?... i see the "manual"
> says 18 ->22 psi front, 20->24 psi rear, but those were bias plies.
>
> I have 175/70 13s.
>
> discussion of trade offs should include such things, ride, handling,
> tire wear, performance, and gas mileage..
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