Warning... lateral thinking ahead...
I've been looking at a lot of info about aerodynamics (specifically mods) lately. Well, (I may just be tired) but it seems like there are two main factors in drag. One (F1) is the obstruction itself (hard to define...) and the other factor (F2) is the after effects (and residual drag) from all obstructions and displaced volume, in the form of a massive pressure differential between the front of the vehicle (relatively high pressure) and the rear (relatively low pressure).
To explain that a little better (and to differentiate between the two), F1 causes distortion by imperfections, and F2 magnifies that by maintaining the pressure differential.
Now, boat tails
et alia address both soft of, but it doesn't directly change F2.
I hope I expressed myself clearly enough... but anyway...
My proposal: A central pressure differential reduction tube. The name is pretty self-explanatory... but it is a tube (yes... a physical tube) that is somewhat central (as practical) to the vehicle then looked at directly on (or at the rear) in the X and Y axes (or otherwise as close to a direct A-B {Peak
High-to-Peak
Low} pressure route as practical-- front to back).
The idea is that instead of more smoothly making air flow
around the very large obstruction (cab) - the total area of the vehicle when looked at straight on - (which F1 addresses directly), it directly addresses the pressure differential (between high in front and low in rear)
through the cab. This would have an artificial effect similar to reducing the area of the cab (reducing the 'void' around which air must flow (F2) vs. simply smoothing the flow around a set void (F1)). This should both increase efficiency, and significantly reduce the residual magnification of the aerodynamic flow factors (making them less important). I hypothesize that, at relative speeds significantly above 0mph, that the effective reduction in cab area will be greater than the ID (inside diameter) of the tube.
Is this too crazy/impractical? Or am I missing something big that makes the whole idea stupid? lol (I've done it before) I will gladly concede that having a tube running front-to-back through the cab might not be the most convenient mod ever... but that is just the very general concept. With modifications for a specific application (to minimize hindrance to passengers, functionality, and so forth), I think it could have potential to hugely affect fuel efficiency. Key word is "think." The actual effect, obviously, is questionable... However. I maintain that it is plausible, and would be a good candidate for someone to try.
Any thoughts? Anyone willing to experiment?