The Camero sits higher on its wheels, than the other cars, so it is just more noticeable. It might be possible to simply lower some polys on the bottom of the car body, to fill the empty space between the wheels, so that you can't see that there is nothing between the wheels. That wouldn't add any polys at least. In any case, it is a very impressive scene.I watched the latest video with your post in mind. I don't think ANY of the vehicles have axles. Or drivers or passengers, either.
It's amazing this is being done on x number of video cards with a lot less RAM than you'd think necessary.