There is a difference between driving with the gate down and without it at all. Has a lot to do with the smoothing of the air flow over the truck and turbulence behind it. The gate down provides more horizontal surface for the air from teh top to become horizontal and results in muc more laminar flow. No gate at all (or net instead) limits the amount of surface the air has to stabilize and thus causes more turbulence and lower milage.
An eningeering student did a test once with a scale model of a Ram in a water flow test. Tested with gate up, gate down, no gate, cap, and simulated cap that goes from top of cab stright downto top of gate.
No gate had slightly more drag than gate up (air pressure built in bed and the rest of the air was able to flow over it and off the back). Gate down was better than gate up, then the cap and finally the sloped cap had best results.