Speed Devils

All cars and tracks:
Press B, Right, B, Right, Up, B, Up during game play.

Unlimited nitros:
Press Down, Up, Down, Up, A, X, A during game play.

Skip current class:
Press Down, Right, Down, Right, A, X, A during game play.

More money:
Press A, Right, A, Right, Up, B, A during game play. (Information from Cheat Machine)