With the help of these achievements you can get one of the following effects:
Cheat mode: Unlock any cheat and the "Cheat Mode" selection will appear at the options screen
Invulnerability: Complete all Grand Prix Championships with gold medals
Runaway: Complete Crash mode with all gold medals on all fifteen tracks
Freerun: Complete the Custom Series Championship the "Freerun"
Face Off: Win all the face offs in the first championship
Two player pursuit mode: Complete Pursuit 3 to unlock two player pursuit mode
Multi-player mode: Complete the first set of Offensive Driving 101 lessons
Unlimited Boost: Unlock everything in the Custom Series Championship
Custom Series Championship: Get all gold medals in every race and win everything in the first championship
Custom Compact car: Win the Custom Series Qualifier to unlock the Custom Compact car
Custom Coupe car: Win all gold medals at the Split Second Grand Prix
Custom Muscle car: Win Pursuit 6 to unlock the Custom Muscle car
Custom Pickup truck: Win Pursuit 5 to unlock the Custom Pickup truck
Custom Roadster: Win all gold medals at Point Of Impact Grand Prix
Custom Sport car: Win all gold medals at the Speed Streak Grand Prix
Custom SUV: Win Pursuit 4 to unlock the Custom SUV
Drivers Ed car: Complete Driving 101 with all gold medals
Gangster car: Win Pursuit 3 to unlock the Gangster car
Hot Rod car: Win Face Off 1 to unlock the Hot Rod car