[Christ]

Originally Posted by bondo

Christ, in cross section these things do happen. Also the proverbial bubble of air is produced in an open truck bed to trick the air in the boundary layer into not entering the bed.

Tonneau covers do improve fuel efficiency. Camper shells do decrease fuel efficiency, as proven in wind tunnels.

The following link has argument sustaining both sides of the camper shell issue. There is a link in the link to a study done in a wind tunnel on how a camper shell is worse aerodynamically but I could not get it to open.

How much does putting a camper shell on a pickup truck impact gas mileage? | Answerbag

Bondo

I should mention here that there were a few assumptions made in that statement, as well:

The cap fits snugly as an extension of the cab.
The truck bed doesn't extend back into the flow aft of the cab roof. (small, short bed pickups)

And, of course, that was not my research showing anything, it was purely intuitive, and explains why people might think that the cap is better than other options.

Take it as you will.