DR I saw a Mythbusters on Discovery Channel and they proved it is a myth with having the tailgate down vs up.

 

my secret for better mileage. . .my size 13.5's flog the go fast pedal on occasion. . .the rev limiter is there for a reason

plus running high RPM's with 1200-1300F EGT's will help burn off carbon buildup on the injectors. this carbon will cause a bad spray pattern giving you lower mileage and lower power. I just look at it as cleaning out my muffler of the soot that has built up