The others have already put forth some very valid C+C.
To add to it myself, the new bg looks great and so does the exhaust fumes, a tip I have for you when window tinting - is to not just darken the window using CTRL+U and tweaking the sliders.. but to also copy the window onto a new layer and experiment with the blending modes (i.e. try out the multiply setting). Doing this sometimes helps to retain the gloss/shine from the window glass.