Here is a another favorite drawing of a car I fixed up in photoshop. The two pictures on the top of in this pic are the two camaros I drew is a tweak fix up I drew and the second one is the original.
I draw my cars by eyeballing everything which includes shading,reflects,and the overall proportion of the cars and trucks.
http://i37.photobucket.com/albums/e60/AnthonysDesign/67camaro_render2.jpg