how do i make particles come off a car's wheels when driving on off road terrain?
Well its more than putting a ParticleEmitter it on the wheels. You should have a touched event on the wheels to determine the part the wheels touched, and then get the surfacetype value of the part they touched to see if any match up to being an off road surface. I love this idea and will try to put an example together for you in this response, ASAP.