Traffic Simulation - Vehicle Elevation

I have a 2d reference file that I am using to establish the traffic lines.  In my 3d model (a SS3 model with mesh components representing surfaces) the vehicles are being drawn at an incorrect elevation, most of them have their tires below the surface elevations.

Is there a particular setting that controls the vehicle offset from the surface?  Should I be using a triangulated surface rather that a mesh surface from SS3?