When you go to the leaderboard there is Car Class X just after R1...How do you get a car in X class?
upgrade it past R1 class
They changed it from Forza 2 now the classes are as follows
Forza 2 - D,C,B,A,S,U,R4,R3,R2,R1
Forza 3 - F,E,D,C,B,A,S,R3,R2,R1,X
You may now upgrade your S car into the R3-1 Classes, and after that you hit the X class or the new U class.
Oh also i forgot to mention the puegeot and acura LMP cars can both be upgraded to X class,
I know because i have them BUT dont buy any race cars until your level 50, or atleast check the list of reward cars, i bought a holden V8 race car only to be given one afew races later
Remove the Restrictors on the R1 Class and you will be upgraded to an X Class.
The #9 Peugeot 908's power goes from 522 kW to 561 kW. ~ 1,200.9 nM Torque to 1,290.1 nM
Quite an improvement I'd say.