At the Create New Driver Profile enter the following.

Code               Result

Onecarefulowner    All cars  
Hellocleveland     All levels 
Rorrimskcart       Mirrored tracks 
Jobinitaly         Mini Cooper  
Offroad            Lancer road car  
Jimmyscar          Sierra Cosworth  
Coolestcar         Ford Puma 

The cheats below are for Time Trial & Single Stage Modes. At the cheat option

screen enter the following.

Rubbertrees        Bouncing collisions 
Easyroller         Monster truck wheels  
Moonlander         Low gravity 
Rocketfuel         Turbo mode 
Prunejuice         Faster game 

The cheats below are for Arcade Mode.

Greatballsof       Shoot fireballs 
Neuralnightmare    Aggressive CPU cars 

Thanks to Revolution reader Colin!