Ever see a semi pulling a flatbed trailer, loaded with plastic pipe going down the road, where the pipe is up above the sleeper height?
If you notice, most of the time they will have the top of that tarped on the front, to keep the air from going into the open ends of the pipe.
That air, going down the pipes, create a greater drag, than tarping it, and preventing the air from entering. I cannot explain WHY, but it is true.
Therefore, I believe opening the windows will have the same effect. It will actually make the problem slightly worse. I think it has to do with the fact that you are disrupting the air flow, and thus creating more drag. Either way, I doubt that it would make enough difference to tell.