I'm in the process of making an Aerocap for my truck. I have done a bit of research and have gotten to understand some basics of aerodynamics. I actually considered this very design, but decided against it on account of it isn't too aerodynamic. It has to do with the fact that the air will not follow that curving slope all the way down. You will get severe flow separation about half way down those windows which will create a lot of turbulence and drag. Hear me now, believe me later, this will save some gas, but only about 5-8%.
The only place you can improve aerodynamics in the back is above the top of the truck bed and try to control the flow there, the sides of the bed and behind the tailgate area are doomed aerodynamically by the basic design of Ye Old Pickup Americanus.
I have a hunch that there is a fairly huge low pressure area created at the in the last 18 inches or so of the truck bed, the air moving along the sides will be seriously sucked into this area and create 2 large vortex which extend back behind the vehicle from the areas above the head lights. The drag created from trying to pull around 2 tornadoes while at highway speeds will be significant. Imagine the amount of energy it would take to create 20 foot whirlwind in calm air, how big a fan would you need? I think this is the amount of energy which goes into creating these trailing vortex.
Maybe if he mounted these vertically on each side of the pick up towards the back.....???? Hmmmmm
