I had someone tell me that driving with the tailgate down would also gain 1 or 2 mpg.
Not sure on that one. It "could" be the case on our older trucks, but I've seen several smoke streams from the labs studying air flow in a wind tunnel and modern trucks lose mileage with the tailgate down or off. It messes up the factory aerodynamics bigtime. On a personal note, my wife's truck when new (2015 Ram 2500 4x4) got a dependable 17 to 19 mpg. When it was about three weeks old we pulled the bed and installed a new CM flat bed. No tailgate obviously and a small bit, around 2" or 3 of headrack and flatbed extending past the cab. With the flatbed induced airstream disruption, mileage dropped 2 mpg and now runs 15 to 17mpg on a good trip when not pulling a trailer.
I think coilover was on the right track in his chasing 20mpg with a straight 8. Now I'm just dreaming here and odds are likely never will, But if I wanted to build a mileage champ AD truck, I'd go diesel. Not Cummins, though I'm a Cummins fan but a Isuzu 4BD1TE. This
details a Isuzu conversion on a Toyota Landcruiser that was seeing 26mpg. Not Stovebolt but interesting for sure.
All the above said, mid teens ain't bad for our rigs. I've got better on long drives but fact is these trucks aren't about modern mpg. For me it's more about spm (smiles per mile) than mpg.