You're right, that the car is always placed on the base level (by default 0, but editable), if no ground object is present. The car can't start in the air. So you need a ground object.. The car then will be placed on the ground, when starting the animation. If that doesn't work, the normals of your polygon object may point to the wrong direction. Just reverse the normals.
Do the following few steps to see its working: Create a new simulation object, create a car object. Go to the Driving Controll tab and Create a ground object. This will create a simple polygon object with a plateau. Move this object to 0,0,0 and run the animation or just step to frame 1.