Could you guys please clarify how a cover would help? I was watching mythbusters a little while back, and they showed how with the tailgate up, it creates an isolated system for air, wich deflects other air from hitting your back end, where as when they put the tail gate down, all the air went over the truck, and landed on the back end, weighing it down and having less MPG.

Though I'm still new at this, wouldn't that theory still hold true for a cover, the air just hitting that instead, putting more force on the back end?

 

A truck, by design, is not very aerodynamic at all. The tonneau helps with that problem a bit.
It doesn't allow air to flow into your bed at all. My tonneau was damaged to the point that I need new rails. Since I haven't had it on, I have lost about 1-2 MPG.
It's not a whole lot, but with a V-8, I'll take any gas conservation I can get.
Hope this helps.

 

buy a honda