I need help with my collision problem

Hello there again, I seem to have a problem with some collisions. In my game I have a car made out of Cubes, but when I place another cube in the world, and I drive upto there with my Cube made car, It just drives into the cube and gets out on the other side. I tried everything, Putting boxcolliders, Rigitbody's on. But it just doesn't seem to work. Please don't say that I have to script it for every cube I put in my world, because I am wanting to go and make a racing track, witch is going to use some stuff I am going to make in Google Sketch Up Pro 8, also if I import thing from there it doesnt seem to collide with my car... Can someone please help me????

All help is appreciagted ;)

EDIT: Now that I have put a Ridgid body on, It seems to have some collision, but the car can still seem to slip through.

Greetz xXZentoRXx