The guys who are telling you to keep it up are right!! It does create a vortex similar to a twister which eliminates the drag. Check out the MYTHBUSTERS on the DISC Ch!!
myth busters did a test both ways . tail gate up won. creating a vortec thing was right.
let it down. there's less wind resistance with it down so i would assume it wouldn't hold the truck back as much. but that just rounds off a small part of the brick :)
http://kwc.org/mythbusters/2006/12/e..._revisite.html) but only because the viewers suggested they test one with a hard bed cover, a mesh tailgate and one with the tailgate removed altogether. The tailgate up was proven again more efficient then tailgate down. Either way, you can't dispute wind tunnel testing (see http://www.nrc-cnrc.gc.ca/highlights...uplands_e.html) and the test proved there was more drag with the tailgate down.
when u have the tailgate down, the air that comes over the cab pushes down the bed of the truck causing you to lose MPG...but with the tailgate up like said before it causes some vortex or wind tunnel in the bed of your truck and actually pushes your truck.
I don't know if lowering the tailgate will help, but removing any excess weight will help. Do you know if the other truck is completley stock?
Has anyone actually tried it both ways with the tailgate up and the tailgate down. DRiving my normal style and got just a hair less on MPG with my 07 Z71. I normally get about 13.5 with the tailgate up and I got about 13.2 with the tailgate down.
Regardless, If you're losing by two truck lengths you're going to need to do something more significant than lowering you're tailgate or taking the tailgate off.
Besides, are you sure his truck is stock or "stock"? Has he had different gears or maybe even a hypertech programmer without you knowing? There's too many variables to pinpoint why you're getting beat...and no its not the tailgate.