Extra cars:
Begin a new race. Select the BEGIN CAREER choice. Submit the name GO_BEARS! (for stock cars) or GO_BRUINS! (for formula one cars) at the "Register" screen. REMEMBER: "_" indicates a space.

In-race car modifications:
After pausing your game during a race. Go to the RACE STATISTICS prompt. Press and hold Circle and push X. A car editor menu with the following options will become visible:

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