Startability up a steep grade is the concern with rear ratio. Around Alabama and the Blue Ridge Hills, you might see an occasional strain to get moving. Other than that, I look at GVWR, GAWR and tire ratings. I like a gear ratio that puts the engine in it's power band on the flats and up the hills at 55-65mph. For me that's been the 3.73 both gas and diesel. Diesel in O/D and direct. Gas in direct and it's first under drive ratio.
My '01.5 250hp Cummins Ram had a 3.55 rear ratio. No problem towing our previous 10,400 lb weighed CAT scale weight, 11,400 lb GVWR high profile 5th wheel. It would downshift on the uphills to run 2600rpm but that wasn't a problem for me. It ran 1750~rpm in O/D. It was rated to tow 9700 lbs.
With the Ford and stock tires, if you're running 1750~rpm at 65mph, probably 3.55. If 2000~ rpm at 65, likely a 3.73. If 2200~rpm at 65, likely a 4.10.
MPH X rear ratio X trans ratio X 336/tire diameter(30.5~) = rpm.
