@linda s I agree with both posts regarding two examples shown. and i agree with the principles you mentioned. Regarding the air deflector on the tractor trailers, that was designed and likely optimized over several iterations. As for our lovely sunraders, i am assuming they were not designed for aerodynamic performance, and if they were i doubt there was much testing and iteration. (if there is aerodynamic testing data with images, i think we need t-shirts made ). On our sunraders, the deflector like geometry just after the cabover section as you move towards the aft, may trip the flow (similar to the first truck image) due to the relatively sharp nature of the geometry as compared to the examples of the car and truck deflector. I'm not an expert in aerodynamics/gas flow or CFD but i have some design experience related to the subject and have encountered similar phenomenon due sharp geometry. Im not positive on the engineering, but i believe the sunraders deflector would cause boundary layer separation and promote turbulent flow. I guess there is only one way to find out!