If I may Domino, I have one criticism of your photoshop technique. Though it's rather insignificant, I think you over use the thin white lines as highlights (i.e. pen tool - stroke path with small white brush to define edges). In some of your renderings (i.e. Your Silvia and Abante) the black lines in the sketches are replaced with white ones in the form of highlights, and they become a little too dominating. A suggestion would be to either use the stroke path - simulate pressure, or to use a large eraser at 0% to gently erase the edges of the white highlights. The best method for defining edges would be to use a lighter shade of the color of your car as the paint color when you stroke the path, instead of white. Or you can reduce the opacity of the layer you used for the highlights to make them less striking.
Obeying the 1-2-3 rule for light and dark would help as well. For example you can create a new layer, and roughly paint the side of your car black or dark grey in that layer. You can then lower the opacity of that layer and it would allow your side to appear slightly darker than the top of your car (hood, windshield, roof), as it would be in reality.
Also I think if you raise the reflection horizon higher to near 1/2 of your car body, it will not only look better but also make for an easier rendering job!
anyways just a thought