i agree with what your saying but not the effect it will have on the body in question, if the smooth bits are extra smooth and the rough bits create turbulence then the speed difference between them will be greater, if you refer this back to the diagram above, this means the car/plane will have a greater aerodynamic influence, but ot say you need the top of the car to 'create drag' is wrong. every moving thing unless in a vacumn 'creates drag'. but that is one thing that designers and engineers will try to lower as whatever the body it will always slow you down and not neccessarily give you any advantage ie downforce