Really like your ideas, but I feel there are a couple of errors..
• Roll bar. It does look to bright to actually be in the car, and behind the windscreen. Also, it would be darker if it were in the car, due to shading.
• The rear wheel is a little off, it has to much positive camber and doesn't look right at all.
• A small one now, but I feel that if the can were really there on the roof, I think that there would actually be a small shadow underneath, or at least shading there.
Otherwise, it is a decent image and looks good

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