We talked about the half-tonneau, but here's something that hasn't been quantified yet.
The manager of this pizza parlor agreed that this box over the tailgate improves gas mileage but he didn't know how much. If you look at the full aeroshells that are truncated at the tailgate, they have a curved top edge about 8" high at the center.
This may be the simplest way to improve mileage. A base plate that prevents the wake from sucking it's way forward into the bed.
https://ecomodder.com/forum/showthre...ies-21204.html