How to tune cars in Forza Motorsport 2?

Before starting a race i want to be able to change the gearing for certain cars on certain tracks, but the game wont let me. The only tuning it will let me do are the tires, but the gearing, alignment, anti-roll bars, springs, damping, and aero wont work. If you know how to fix this please help me out.|||to tune any part in the game, you have to have a tunable part installed on that car. when you buy the parts, it will tell you if it's tunable or not.