Highly debated topic on the old forum.. I believe there was not an increase when a camper shell, tonneau, or tailgate left down. If you put a piece of trash in your bed, does it fly out? no, it stays in there for the most part. there is a pressure bubble of some sort that forms from the air coming off the top of the cab, and putting a tonneau cover or leave the tailgate down actually messes that up, and decreases mpg. I'm sure if you did a good search on the old forum you'd get a better answer than that. I think the majority of the info was on the (now dead) ttora-ne.com site...TuRD03 said:Just wondering if its worth shelling out the cash for? What kind of mpg increase did u see?