Best I have ever done with my stock '01 CTD, auto, 4:10's was 19. I was really babying it too. Boy Scout trip from outside Sacramento to Tillamook, Or. So that 19 at freeway speed(65 or 70mph speed limit, always staying within 5mph of that). included flatlands thru the upper central valley, and mountains thru Shasta and the Siskou's.
I do fair trailering my tractor at about 7000lbs total. But WOW, what a difference towing my 5th wheel(27' 7000lb) or Boy Scout Troop trailer(14' enclosed 7000lb @~5000lbs). The 5th wheel and Scout trailer have big square bodies; I have actually though about finding a used Sprint Car wing for the truck
Mine is the early 235hp motor. Even with the lower power(it is still stock except BHAF, and 4" exhaust), the 4:10's have never been needed. I would not get those low gears again. If budget ever allows, I'll change it to 3:73.
I did find, that I got ok mileage even with the 5th wheel up to 62mph. From there it fell to pieces. We went from here(30 miles east of Sacramento) to Corpus Christi, Tx two summers ago. Ca is 55mph towing speed limit state wide. Having got a ticket once towing my tractor at 65, I stick with the speed limit or close to it. Getting in to Az, NM, and Tx though, you could tow at 65 or 70. The 5th wheel towed like a dream at 70-75, but the fuel mileage , like hit a wall at 62mph. I could run 13-14mpg on the flats up to 62mph. Then it dropped like a rock.
And this last time I filled up, was all towing and or hauling wood. Trailer @ 7000-9000lbs, and several loads of firewood @ ~4000lbs. That fill up was 11.3MPG. And I never reset the computer from before, so I dont know what it would have said.