Fly the tank
First put in the flying car code (Right, R2, Circle, R1, L2, Down, L1, R1). Then get a tank or put in the code (Circle x6, R1, L2, L1, Triangle, Circle, Triangle).
Turn the gun backwards, keep shooting the gun. You will take off and fly for along time if you keep shooting.
More tricks, tips and cheats for this game are right here - Grand Theft Auto III cheats
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