I don't have a tonneau cover but I do have a camper shell.. I also read where camper shells would help fuel mileage but after taking four grown men to lift this thing up ,I'm saying no way it could help fuel mileage.. I'm sure just the cover is not near as heavy as my camper shell.. But anyway, I think you will love the cover for its purpose. I can't imagine not having this shell to keep all my hunting gear and tools dry and out of sight...