Unlimited turbo:
At the main menu, press Down, Down, Down, L, R, Z.
No boundaries:
At the main menu, press Left, Right, Z, Left, Right, Z.
Low gravity:
At the main menu, press Left, Right, Up, Down, B, B, B.
Big heads:
At the main menu, press B, X, R, L, Right.
Bouncy bike:
At the main menu, press Up, Up, Y, Y, X, X.
Tag mode:
At the main menu, press Z, X, Z, X.
Tiny player:
At the main menu, press L, Z, Left, Right, B, B.
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Jeremy McGrath Supercross World Cheats, Codes & Walkthrough/Guide/FAQ for Gamecube from Cheat Code Central
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Jeremy McGrath Supercross World Cheats at CheatingDome.com.