Jeremy McGrath's Supercross World Cheats

At the main menu enter left, right, up, down, b, b, b.If done
correctly, at the bottom left hand corner it will say, "You Feel
Lighter."

Successfully complete the hardcore part of career mode to unlock all bikes.

Infinite turbo
At the main menu, press Down(3), L, R, Z. If you entered the code correctly, the lower left corner of the screen will flash and the message "Need for speed" will appear.

Bouncy bike
At the main menu, press Up(2), Y(2), X(2). If you entered the code correctly, the lower left corner of the screen will flash and the message "Bouncy, bouncy" will appear.

Big heads
At the main menu, press B, X, R, L, Right. If you entered the code correctly, the lower left corner of the screen will flash and the message "Your head expands" will appear.

Tiny mode
At the main menu, press L, Z, Left, Right, B(2). If you entered the code correctly, the lower left corner of the screen will flash and the message "Drink me" will appear.

Tag mode
At the main menu, press Z, X, Z, X. If you entered the code correctly, the lower left corner of the screen will flash and the message "Tag you're it" will appear.

Low gravity
At the main menu, press Left, Right, Up, Down, B(3). If you entered the code correctly, the lower left corner of the screen will flash and the message "Feel lighter" will appear.

All bikes
Successfully complete the hardcore part of career mode to unlock all bikes.

Press L, Z, Left, Right, B, B
Source: GamePro