Ive created a shadow catcher to suit an HDRI image. The shadow catcher is only basically doing ambient occlusion and there is no signs of shadows. There should be an entire shadow under the car.
Try this setup: I only changed the diffuse color and the white handle on the color ramp. I also had a downwards facing sun lamp. Which, mind you, will ruin the shadow catcher if you rotate it beyond a certain point, you can play around with that and try to compensate those two values I changed to your own scene.