Type at the main menu:

 ICANCHEAT       Cheat mode ON
 CLIPPED         No clipping mode
 RECKLESS        Reckless mode

 Type at the track selection menu:

 COMEPLAY        ?
 LAPSET          Set # of laps
 FREERIDE        Pepsi Planet Track Available
 FREEDRIVE       All Tracks

 Type at the car selection menu:

 TAKEMEON        Champion Motors Sponsor Selectable
 WIDEBODYOPEN    Speed Demon Paint Job
 WILDONEOPEN     Wraith Car Selectable
 REDFURY         X-Wave Car
 WILDONE         Jeff's Car (select by changing sponsor)

 Type the cheat at the menu and press [Enter] and you will hear a
 voice as confirmation.