Change Difficulty 
     To outfit machine guns and rockets to your car, go to the Options
menu,  and select Options: Cheat Codes. Enter the code howhardcanitbe,
followed by a number 0-9. The most difficult is howhardcanitbe9, while
the easiest is howhardcanitbe0. Enjoy.
 
Machine Guns and Rockets 
     To outfit machine guns and rockets to your car, go to the Options
menu, and select Options: Cheat Codes. Enter savethekids as a cheat to
hook your ride up with machine guns and rockets. When playing the game
press  R3 (click the right analog stick) to fire rockets and L3 (click
the left analog stick) to fire machine guns.
 
Unlock LAPD Car 
     To unlock the LAPD car, beat all circuit races in LA.
 
Unlock Parisian Police Car 
     Beat  all  circuit  races  in  Paris  (Arcade Mode) to unlock the
Parisian police car.
 
Unlock Tokyo Police Car 
     To  unlock  the  Tokyo Police Car, beat all circuit races (Arcade
Mode) in Tokyo.