Jeremy McGrath's Supercross World - Platform: Gamecube

Infinite Turbo: 
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At the Main Menu, press Down, Down, Down, L, R, Z. 

Low Gravity: 
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At the Main Menu, press Left, Right, Up, Down, B, B, B.

Moon gravity:
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Press Left, Right, Up, Down, B(3) at the main menu. The lower left 
corner of the screen will flash and the message "Feel lighter" will 
appear to confirm correct code entry. Repeat the code to disable its 
effect.

Bouncy bike:
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Press Up(3), Y(2), X(2) at the main menu. The lower left corner of 
the screen will flash and the message "Bouncy, bouncy" will appear 
to confirm correct code entry. Repeat the code to disable its effect.

Big heads:
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Press B, X, R, L, Right at the main menu. The lower left corner of 
the screen will flash and the message "Your head expands" will appear 
to confirm correct code entry. Repeat the code to disable its effect. 

Tiny mode:
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Press L, Z, Left, Right, B(2) at the main menu. The lower left corner of 
the screen will flash and the message "Drink me" will appear to confirm 
correct code entry. Repeat the code to disable its effect. 

Tag mode:
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Press Z, X, Z, X at the main menu. The lower left corner of the screen 
will flash and the message "Tag you're it" will appear to confirm correct 
code entry. Repeat the code to disable its effect. 

No boundaries:
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Press L, R, Z, L, R, Z at the main menu. The lower left corner of the 
screen will flash and a message will appear to confirm correct code entry. 
Repeat the code to disable its effect.