Enter these codes on the Main Configuration screen as the Player Name:

lovebun		secret car
mayte		secret car
tinkle		secret car
suicyco		secret car
2x4b523p	secret car
formula 1	second level cars
superman	destruction mode
dr death	invincible human cars
goldboy		premier cup
mreprise	bonus cup
cup won		end sequence
i won		win race
roll em		credits
duel		hard opponent
toptunes	cool sounds
cinema		widescreen
remove		remove cheats
tachyons	speed up game
yotarace	increase size of track
freaky		wacky color scheme 

Thanks to Revolution readers Sebastien Lehmann!