PSI right it's a system lifted off Forza 2....being a forza 2 guy I'll explain how it works as I've noticed a few people not quite getting it.
Now first apologies I've just come back from the wigan beer festival so I'm a little pished LOL
The number that is being talked about is basicly an overall performance rateing for your car. Say you have a car and stock its rated at 500 points, add R1 tyres and that rating jumps to 600 points, Boost the BHP and it jumps again to 650 points, ok downgrade the tires to say R3 and it comes down to say 610 points. Every aspect of tuning changes the points value...ok so far? Good! LOL
OK take a race, online or off line , rules stipulate you can enter ANY CAR, but its performance points can be no more than say 550 points. your now left with a problem. do yo take a stock car who's points are close to 550 points or do you take a lesser car and tune it upto 550 points for the race? Now heres the clever bit! OK you can go upto 550 points for the race, do you use the available tuneing points for handleing? or power? or a mixture of both?
This comes down to 2 factors, your driveing style and the trck! It makes for some pretty specific tuning which is limited, rather than just max your cars tuneing in all sectors. this also makes for some super close racing, as all the cars in a race are limited to a max of 550 or whatever the limit is. This really made the tuning a game on it's own on the 360 Forza 2 game and will do the same on GT5p.
Cant wait for some online races based on this system, there really going to be close with guys in pretty ordinary cars tuned to the eyeballs running the same times as guys in stock ferraris and astons! If you havent experienced this system on Forza 2, DONT WORRY it works fantastically!
Again sorry I'm pished LOL