Bonus cars
Start a new race and choose the "Begin Career" selection. Then at the "Register" screen, enter "GO_BEARS!" (for stock cars) or "GO_BRUINS!" (for formula one cars), where "_" indicates a space.

In-race car modifications
Pause the game during a race and enter the "Race Statistics" screen. Then, hold L1 + L2 + R1 + R2 + X + Circle + Select. Now a car editor menu with the following options will appear:

 Drafting effect
 Draft distance
 Front downforce
 Rear downforce
 Fuel consumption rate
 Incremental braking
 Tire wear rate
 AI speed percentage (50-150)
 Torque spins
Centrifugal force