Use the following trick to get free parts without completing the game. Go to 
"Garage", and select "Customize Your Car". Go to "Performance Parts", and press 
X to auto upgrade. This will check off all parts you can put on. Do not press A. 
After you press X, press Xbox 360 Guide. Go to your friends list, and join 
someone's session. Note: You can also go to the recent players list for a better 
chance of finding someone playing online. When asked if you want to start a 
multiplayer match, select "OK". When you get on the street, you will have the 
parts installed with no money deducted from your bank. This also works on any 
interior or exterior part, but the process is slightly longer to make the car 
you want. For example, highlight "Front Bumper", then press X. Go to the bumper 
you want, and press Xbox 360 Guide. Join someone's game, and when you roll out 
of the garage, you will have that bumper. Quit the multiplayer game, return to 
the garage, and get any other part desired by doing the same thing.