The best psi is per tire/wheel/suspension/load/car...

Best thing you can do is take it on a track(or skidpad), get a tire temp guage, and do 2 laps.. or something to get them warm. Then take the temp of the tire, if themiddle is hotter than the outsides, you've got too much pressure, if the outsides are hotter than themiddle you need to bump it up itll its level accross the board, but if the outside edge is hotter than the inside edge, you need to give the car more camber, if the inside edge is hotter, you need less camber (unlikely in a street car... lol). (you'll be temping the actual tread.. not the sides of the tires)

Or another way is to do a quick burnout. Look at the tire marks, if the inside is thick and the outsides are faint,turn the pressure down, if the outsides are thick and the insides are faint, bump up the pressure.

This is assuming you want best grip. If you want best grip, and decent streetability, note your tire pressures (somewhere between 35-45.. my best guess would be 36-40.. but this all depends on the tire and car options.. the heavier the car, the more pressure is required..) and bump it down a couple psi so you get a decent ride (feeling everything on the road just isnt fun..)

Another option is to get chalk and make chalk marks 1-2in wide. about 4 or 5 of them around the diameter of the tire, from 1in in the tread to 1in in the sidewall of the tire. (for simplicity, just do the side of the tire facing out) do all 4 tires, and take the car for a "hotlap" an empty parking lot with NO POLES would be a good place, just do a couple of circles, going pretty fast.. Be responsible here, dont get close to potholes, bushes or anything that could possibly hurt the car.. driving it hard will not hurt the car.. if your cars solid..
Anyways, inflate the tire extra, and deflate the tire untill all of the tread is being used.. If the chalk is worn off on the sidewall a little bit, inflate it some more, if you have some chalk on the tread, deflate the tire a couple psi..

Tire pressure is a black magic.. and can make or break handling.


KEEP IN MIND..all the above isfor maximum grip in response to speed racers post.... (im sorry if i've rattled on.. but thats how I inflate my tires..)

also running max psi on the tire will probably just give you uneven tire wear... continue reading.

If you want just daily driving inflation.. go HERE just jump down to the bottom for the how to.>>>>>http://www.thedieselstop.com/content...re%20Inflation

 

Lower tire pressure will give you more traction off the line but at the same time will increase rolling resistance. To launch an AWD turbo car it is sometimes smart to over inflate your tires, this will increase wheel spin on the launch keeping our cars from bogging. Believe me, tires are cheaper and much easier to replace then a clutch.

 

Thanks Andyrew Im going to try the chalk thing as soon as I get a chance.