To unlock these body parts, you need to complete certain things in the game.

Unlockable	How to unlock
B-type body	100 network victories
B-type tire	First network victory
B-type wheel	Win in 10 seconds
C-type body	10 network victories in a row
C-type tire	Drive 10 billion miles
C-type wheel	Defeat 100 enemies on your wall
D-type wheel	Boost 5 times in one round
E-type wheel	Jump 5 times in one round
F-type wheel	5 network victories in a row