Shifting Mystery

Seems like only a fool would say it, but here goes, "I Love my truck." So, as I shift from town to town, a mystery picked up speed. This little bugger (4 wheel drive, 5 speed, 5 cylinder ) had been averaging 25-27 mpg so to class the mask up, I added a tonneau cover. The advertising claimed it would add a ten percent increase in mpg, but I wasn't fool enough to even anticipate that outcome. 
Reverse engineering! The miles per gallon went down to twenty four (what kind of a deal is this?) Today, the painter in the postoffice parking lot told me he smelled gas. We investigated, and sure enough, my love was leaking that precious commodity.