I need help on how to make a car cake look like it is sitting on the wheels, with a gap underneath!!!  I would be very grateful for some assistance/ideas on how to achieve this.

ASimply, use a board (wood or plastic, not cardboard) same size as car, and use small blocks that can be hidden by the wheels. I've done cars before and carved a bit from where the underside of the car would be - not too much as to upset the structure of the cake - and used a band of black fondant to look like shadow.

A piece of foam underneath that will lift the car up and small enough you can't see it without really looking.  Could even be a block of thin wood. 
 

I used styrofoam under my truck cake. I painted it black like Mike Mccary's dvd recommended. It worked great. Had no problem putting wheels on . hth