[Christ]

Bondo -

To clarify a bit on the cap/no cap/tonneau thing:

When you look at a vehicles frontal area, you have to look at the largest cross section of the vehicle. For best aerodynamics from a frontal view, you want the largest cross section to be as close to all in the same area as possible. Refer to tear-drop profile - The largest cross section of the frontal area is all in one place, close to the front of the shape, with a heavy curve reaching it from a low stagnation point.

The same idea applies at the rear. When considering the cross-section of a trailing wake, the wake occurs at every place that the flow isn't attached. This includes the rear window of the pickup truck. The tonneau reduces the effect slightly by giving the flow a place to smooth out before running off the back of the truck into a new wake area, and the camper top helps the most because it moves the entire wake to one area.

If you imagine that each step takes X energy:

Open bed = detach at the roof, circulate in the bed, flow over the tailgate, detach at the tailgate edge

Tonneau = Detach at the roof, reattach (maybe) at the bed cover, detach at the tailgate edge

Camper = Detach after the camper.

It's not a perfect representation of the way things work, but it should give you some idea as to why a camper top is better (but not universally, of course.)