I have a cap, and honestly maybe only see a 1/2 mile per gallon with shell. I prefer the tonneau (hardshell) over the shell cap myself. I have both, but since this shell was on the truck when I bought it, and the snow came too soon to change to the tonneau. I'll drive with the shell till spring. Besides, its snowing or was, and the shell keeps everything so nice and dry, where with a tonneau, you get snow inside almost everytime you open it. Well, we'll see with the 4.6 in the spring how the mpg goes, tonneau vs shell.
In my 90 F150, 5.0 AT, with the tonneau, mpg went up over 2 mpg over the shell and 3 without anything on back