There is a University of Texas PDF/Graphic often cited in the Ecomodder forum illustrating the benefits of pickup truck tailgate up Vs down, roof wing, partial rear half bed cover, full tonneau cover and so on.
The long and short of is the rear half cover (
dead air pocket) provides nearly the same benefits of a full bed cover.
Aerodynamics - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
I'd find and post the PDF for you, but I'm signing out.
Maybe later or just search it out for yourself.