Since I've been asked a few times today, here's how to figure your points to determine your class:
Base class - Find your car by year and model. These are your base points.
Tire size points - 1 point per mm over 205. This is for front and rear. If you have 225 fronts and 255 rear tires, you have 70 points. [(225 -205=20)+ (255-205=50)=70 points].
Tire treadwear points and all other modification points
Example:
1988 944S -
Base Points - 250
Tire Points - 80 (245 front/rear)
Mod Points - 140 (treadwear 100 - 40 points, sway bars 20 points, camber plates 20 points, springs/torsioin bars 20 points, weight savings 40 points)
Total points - 470
2006 Boxster S
Base Points - 415
Tire Points - 100 (235 front/ 275 rear)
Mod Points - 20 (treadwear 20 points)
Total points - 535
The link to the 2012 rules is below. Driving classes and modification rules start on page 14. Models and base class points start on page 48.
http://zone8.pca.org/rules/2011/2012Z8Rules.pdf