I've heard that a pocket of air pressure is created in the bed of the truck with the tailgate up. This is suppose to be helpful. When you lower the tailgate, it's supposedly creates more drag and you get worst mpg. I have tonneaus on all my trucks though and that seems to help a little bit. What the heck. It's just money anyway and I'll drive just like I do now at $1.79 a gallon or $2.50 a gallon. It doesn't matter. Just have to eat one less cheeseburger I guess.