Believe it or not on the myth busters they proved that the dropping/removing of the tailgate is negative on fuel consumption...the cab of the truck creates a vacuum and the air travels over the tailgate (this is why trash and dirt and stuff end up blowing up and hugging the bed closest to the cab, its being sucked towards the vacuum)
http://kwc.org/mythbusters/2005/11/e...s_cures_f.html
I think at lower speeds it might help, but highway speeds its negligable
I found that if I keep my tires at 35psi I get ok gas mileage...but it rarely changes
Brian
AwPhuch