As you progress through career mode, you'll unlock a number of different parts for your ATV/MTX. These don't necessarily upgrade your vehicle so much as modify it, diminishing certain stats while upgrading others. It certainly won't satisfy fans of Forza-style tuning nor does it add an obscene amount of depth, however it does ensure ensure that your ride is tuned to your driving style for maximum effectiveness. http://naildbiter.com/DU7Q8