I had found a college that had done some wind tunnel testing with respect the tailgate up/down , bed covers and shells but can't find it now. But from memory, with the tailgate down, they measured ~5% increase in drag, with a bed cover, they measured ~15% decrease in drag. They went on to figure out what was the best shaped bedcover/shell and they found that if you had one that tappered from the top of the cab to the tailgate with a rounding effect just as it approached the tailgate was best. However, it didn't look like a very functional design. I'll see if I might have that bookmarked at home when I get home Sunday.

From my experience and reading other peoples posts/findings, screwfun pretty well sums it up and is what I pay attention to.

Cheleri

 

Check out http://mars.acnet.wnec.edu/~ehaffner/did.htm for the skinny of tailgate up/down/bed cover/shell effects on mileage and preformance.

Cheleri

 

For what it's worth, I remember hearing stories about guys running with their tailgates open, and having them come off! I guess if you hit the right bump and the gate bounces up enough to disengage the one side, you could wind up with it dangling from the cables. Not cool. Also, if you stop to eat somewhere and leave it down, it's much easier and quicker for someone to rip it off. Then you would finish your trip with no tailgate and could do a milage comparison!