The whole up/down debate goes back waay before mythbusters. I can even remember those wackos Click and Clack talking about it a long time back. The reason for the advantage of tailgate up should not vary from truck to truck.
Essentially, if you imagine a line from the top of the cab to the top of the tailgate, the air inbetween becomes trapped and forms a cushion of higher pressure air. Therefore, air flowing over the top of the cab rolls off the top of this cushion of air and over the top of the tailgate. The end product would make the air going over the truck look more like that going over a wing.
If you put an airgate on and by extension lower the tailgate, you loose the ability for that cushion to be retained and then have a turbulant mass flowing over and into the bed of the truck, increasing drag.
I'm not going to argue with someone who says they get better efficiency with the tailgate off. Perhaps your mechanic slipped a "vornado" or "turbonator" in while you weren't looking.
Essentially, if you imagine a line from the top of the cab to the top of the tailgate, the air inbetween becomes trapped and forms a cushion of higher pressure air. Therefore, air flowing over the top of the cab rolls off the top of this cushion of air and over the top of the tailgate. The end product would make the air going over the truck look more like that going over a wing.
If you put an airgate on and by extension lower the tailgate, you loose the ability for that cushion to be retained and then have a turbulant mass flowing over and into the bed of the truck, increasing drag.
I'm not going to argue with someone who says they get better efficiency with the tailgate off. Perhaps your mechanic slipped a "vornado" or "turbonator" in while you weren't looking.
