I hadn't noticed, but that graph has the half-tonneau less effective compared to the full tonneau, then the wing and cover is more than twice as effective. I question htat.
Quote:
Drag reducing partial tonneau for pickup truck
Abstract
An aerodynamic drag reducing device for a pickup truck includes a tonneau cover which extends between the side walls of the pickup box and forwardly from the tailgate to a distance short of the passenger cab so that the tonneau cover encloses a rearward portion of the pickup box. Tonneau covers enclosing the rearward portion of the pickup box to a range of approximately 40 to 60% of the total length of the pickup box provide a greater reduction of aerodynamic drag coefficient than obtained with a tonneau cover enclosing the full length of the pickup box.
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https://patents.google.com/patent/US4573730A/en