Our new truck doesn't have a tonneau cover for the bed, and we're thinking that it would probably improve aerodynamic performance (and mileage) if it did. Does anyone have experience with adding one ...
I have a hard roll type cover (Pace Edwards) on mine. Can't say I saw any change in mpg either way.
But I did not buy it for that purpose. I keep various things in the bed, towing and not towing, that I can close the cover, lock it and the tailgate, and keep them relatively safe from prying eyes.
When the 5er is attached, I just pull it back near the hitch. When not towing I pull it all the way back covering the hitch.