Extra cars
Start a new race. Choose the "Begin Career" selection. Enter "GO_BEARS!" (for stock cars) or "GO_BRUINS!" (for formula one cars) at the "Register" screen. Note: "_" indicates a space.
Wire frame mode
Enter "WYRED" at the "Register" screen.
Wide cars
Enter "PHATCARS" at the "Register" screen.
Press Start car
Enter "PRESSSTART" at the "Register" screen.
In-race car modifications
Pause the game during a race. Highlight the "Race Statistics" selection and hold [R] + [C]. A car editor menu with various options will appear.