Hi Andy, it all depends on the location and the type of shot I want to get. Most of the 3/4 jump or sideways shots I take are taken at around 1/250 - 1/500 depending on how fast the car is travelling. I'll then adjust the ISO to achieve an aperture of around 7.1, which is the 'sweet spot' for most lenses and will keep the whole car sharp. The shutter speed will ensure some blur in the wheels to give a sense of motion.
For some shots (#4, #6 & #16 above for example); I'll stop the lens right down to f/2.8 to achieve a very shallow depth of field in order to isolate the subject. Sometimes this means a much higher shutter speed but other factors provide the sense of motion (dust or mud usually).
Hope that helps a bit.
Rhod