the wheels seem to be suspended because it is the shadow that makes this effect but still not quite over'm trying to do some good settings for the materials and for the wheels I used rubber ..
The specular on the tires does look good but I suggest you put a darker, almost black, diffuse color in the rubber material to make it really look like tires material. And maybe some texture for the side walls.
I'm tempted to move this thread into the W.I.P. section as it's clearly not finished,... I understand you are still working on this?