I don't know about all the computer modeling science and stuff, but the tailgate being up doesn't seem like it should add any 'drag' to a truck.. and here's why.
When you drive, the air hitting the front of your truck goes up and over the cab.. the cab deflects the majority of air out of the bed.
To further illustrate my theory.. when I have a load of loose garbage in the back of my truck and it's windy out, I 'point' the nose of the truck into the wind when I park it.. and stuff doesn't blow out.
If anything, you should have kind of a vacuum as the air rushes OVER the bed.
Just like the idea of driving close behind a big tractor trailer on the highway.. also called driving in their 'slip stream'.. the tractor trailer deflects any air from hitting your vehicle, reducing its drag, and the air rushing along the sides puts you in sort of a vacuum.
