If you want realism and your levels aren't going to be totally plane, I suggest you to move to Tokamak. It's easier to implement that with its physics engine. You also can program your physics routines, but you will spend a lot more time, and will have to solve future problems like what happens when the car flips, or the problem that you explain.
Try Tokamak, now there are a lot of documentation that explains all about it (well, almost all).
Regards
Try Tokamak, now there are a lot of documentation that explains all about it (well, almost all).
Regards