Here is a proven theory on tailgates up and down or covered:
Tailgate up = the air flows over the top of the cab, hits the tailgate and starts to "roll"... as the air "rolls" it forces the air coming over the cab up and away from the tailgate, this is proven by driving in the rain.
Tailgate down = the air flows over the top of the cab, and out the back of the truck, where it creates the draft, that the daft like to linger in... the draft actually pulls on the back of the truck, slowing it down or making you use more gas pedal to overcome the suction.
Bed Cover = You can gain some fuel economy by covering the bed, as the air flows over the cab, starts rolling right behind the cab, which forces the airflow to go up, rather than down, which eliminates (somewhat) the suction of the draft.
