MotorheadG 30

To be more specific...for every 8 wins you get in any single player race, you will unlock a track or vehicle in alternating fashion. If you are going for the full 200, beat all the challenges first, then do a bunch of custom matches:
With shadows off
Against rookie AI
Using only the cannon for weapon
Keep the mode the same but change the number of points to win from 15 to 5
It should only take an hour of grinding if you have already completed all three levels of challenges
(Thanks to TerreBullet from xbox360acievements.org Achievement Guide and Roadmap for all information)
You can just keep hitting retry and it does count BTW