I often struggle with composites and find them hugely difficult to get the correct angles and perspectives, but I have found that shooting for the backplate (rather than finding the right backplate for your shot) proves far more successful. This also giving you more insight into how and where to light the car accurately for the environment.
For this 1970 Dodge Challenger, I used a Nikon D800, a 17-35mm lens, circular polarizing filter, and a Litepanels Astra for light-painting.
The light-painting itself was completed pretty quickly in around 30 minutes, during which, I used a jack to raise the wheels slightly. This, and a slow shutter speed, allowed me to get a more realistic spinning blur on the wheels instead of relying on Photoshop for the effect.
Overall post-processing took around 5 hours or so and, once completed, consisted of 107 layers including the vehicle layers and numerous contrast, curves, blurs, and reflection layers, among others.