steps for you
1) open the render settings window and set the renderer to Mental ray.

2) go to the Indirect lighting tab and create an Image based lighting node.

3) In your scene scale the IBL sphere to size surrounding your car and encompassing your scene.

4) with the sphere selected go to the attribute editor and in the field marked Image name browse to your HDR image.

5) create a plane for your ground and assign the Use Background shader to the plane.

6) select the plane and in the attribute editor go to the useBackground 1 tab and set the Specular color all the way left to make it black and reflectivity and reflection limit to 0

7) go back to the render settings and in the Indirect lighting tab turn on final gather.

8) go to the common tab and under render options turn off Enable default light .

9) set your render size and camera ect and render then save file using a format that will allow an alpha channel ..ie targa

You can render without primary visibility turned on for your environment if you want to use a low res HDR and comp it later