[PaulB2]

I'm guessing this effect is pretty miniscule, and anyway if there is a pressure differential across the tailgate, the same pressure differential exists when an intermediary form such as a box is there, and has the same effect.

The back of the vortex "sucks" the tailgate (or whatever) forward, and the front of the vortex "sucks" the front of the pickup box backward, so all that would seem to cancel too.

I'm guessing also that if you have a round vortex in a square hole, the main vortex may be streamlined but there might well be mini counter-rotating vortexes and other turbulence in the corners. If we take care that the outside of the vehicle is streamlined to avoid turbulence, maybe we should in that box too.

I'm just wondering if anyone has investigated along these lines.