I cant say for sure but to me it looks like you have an issue with your ground mesh.

I have done this before where you put 2 ground mesh's together and the edges don't align exactly.

That's where your model can fall through like what your picture may be showing.

You car may be hitting an invisible rigid body model too.

The best idea if you want help, is to upload your project for us to look at ?

If you have custom models, you may need to include them so we can get an exact view of the problem.