Thank you, that solved the problem. Most of the parts were components, so I did as you said and exploded all of them and made them one single group, then set it to ignore. I'll just keep the bumper as a solid and maybe add some invisible walls on the sides and rear to add some collision.
About readback collision geometry, how would you fix that if I ever came across such a problem? With the first model that I had that problem when I turned readback collision geometry on and noticed the wiremesh was not consistent with the actual tire model, it seemed to be floating a bit higher than the tire itself. Would it be a matter of just exploding and re-grouping the model in question?
Thanks once more for the time you've taken to help and for developing such a great program for Sketch Up users. I am enjoying it very much.