My car wheel sinks quarter into the ground

I have been messing with Unity3D. Imported a car from sketchup and added box colliders top and bottom and included 4 wheel colliders. However during play, my car wheels sink into ground. I maded wheel colliders big. It still sinks. Not sure, what is happening. Would appreciate, if you could have a look and guide me.

I could not upload my project because it is 43,396KB and is too big. So I registered myself at dropbox to create this link https://www.dropbox.com/s/5fo59zsogulfqo3/MyTrial%20-%20Copy.zip

Please help. Strongly motivated to do a car game and stuck. Just trying out different things and learning and need some help

Thanks