Infinite turbo
At the main menu, press R2, L1, Square, Circle(3). If you entered the code correctly, there will be a quick flash indicator in the bottom left-hand corner of the screen.
Low gravity
At the main menu, press Up(4), R1, Square, Circle. If you entered the code correctly, there will be a quick flash indicator in the bottom left-hand corner of the screen.
All bikes
Successfully complete the hardcore part of career mode to unlock all bikes.