I have a full bed tonneau cover, but TBH, not sure, maybe a 1 mpg increase, but I put heavy Falken Wildpeaks on truck so any gain was a loss. The cover more of a convenience in keeping stuff partially secure or dry from the weather. But I do not have a heavy diamond cover that can take weight on it, I have full tonneau fiberglass which does well to keep items in bed dry, since it overlaps sides, front and over tailgate.
CAVEAT: If you want a cover, get for convenience and purposeful use, not for a mpg addition. ALL IMHO...
View attachment 557192View attachment 557193View attachment 557194