I remember a while back, Myth Busters tested the fuel economy of a Dodge Dakota several ways to try and de-bunk the myth that an open tailgate delivers better MPG.
I paid attention.
The truck was tested four different ways:
-Tailgate up
-Tailgate down
-Tailgate off
-Tailgate off, and an aftermarket net installed in its place
Tailgate up delivered better MPG than off or down, but to my astonishment, the net was the best of them all!