Originally Posted by dodge man
The myth busters show pretty much proved that it actually hurt mileage, although not very much. Same thing with the tail gate off or down, that hurts mileage. Those mesh gates and what not hurt the mileage even worse.
Take most anything Mythbusters do with a grain of salt. Most test are just seat of the pants and not scientifically controlled so results can be very misleading.
There have been a few studies on this and the results seem to show a positive result. Even Ram engineers have said a cover reduces drag, reduced drag equals increase in mpg. My take would be a very light cover may give you a very small increase in mileage, but a heavier cover would subtract from any gain in efficeincy.
Wish it would make a couple more miles increase, but it seems if any increase is there it is very small with the right cover.
The positive with a cover is that people can't throw any garbage in the bed of your truck. That will keep the load a little lighter and increase mileage.