PT2

Right, There's not much to doing this really.
It's one of the easier things related to car physics
And can be done using CarEd (or any 3d program that can models in '.asc' format), and Plaything 2.

-This first thing you'll want to do is open you car's model in your 3d program of choice
(CarEd or what have you) and create a new object that surrounds the car in it's own
basic shape (besides the wheels) as shown:

-save this new object as a new file (seperated from the car model) and export it as an
asc file.