Hi
I try to write a physics car use a Wheel Collider. I had a simple car controller ( only moving forward ).
Now i have problem. My car turn left ( it looks like it rides a large radius circle ). I think that the one of the wheel run a bit slower than the other one.
Help
Regards,
Leszek
Perhaps you could follow my tutorial. There are of course others all over the place on the forums, documentation and wiki. 
Control your suspension parameters for all of your wheels and look if the wheels are placed exactly at the correct positions.
Also Take a look at my little car thingy
http://forum.unity3d.com/threads/88143-Me-attempting-to-achieve-some-semi-realistic-vehicle-physics-with-wheel-colliders.