Well, that depends on many factors. But I think the optimal is around 15000-25000 polys (including wheels), for a high res version.
1000 cockpit outside view (and that's even too much)
1000-1500 x 4 for the wheels. But it's better to use less polys here and use them in the body
Apart we have the body details (wipers, mirrors, etc), driver and the collision model.
And then 4000-5000 cockpit in the inside view, but this number may vary and is not so important, as the player just load one cockpit. In some of the cars we're creating from scratch we have very high detailed cockpits with around 20000 polys, and rF can move them smoothly without problem. But it's better to optimize that as much as you can.
Hope all this will help
BTW, are you planning to create a car? if it's something historic don't hesitate to contact us if you want to collaborate with us