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Aside from the obvious variances between the lighting of the added object and the background, as well as shading around the vehicle to help it better blend into the background, one thing I noticed that is really throwing off the image is the scale.  Considering the perspective of the original background, you almost cannot expect to fit that car into view unless you scale it as you have, but it needs to be placed "deeper" into the background.  This means the actual perspective of the car still needs to be tweaked to make it work at all.

Not only were both images shot at different camera heights and angles, but focal lengths as well. Focal length has a direct impact on perspective, especially for objects in the foreground.  The closer the object is to the camera, or the more significant it is in the picture, the more the impact will be seen.  This directly affects any objects you may want to add into the scene.

Edit: Iamsam essentially noted the same thing as I was posting this.