I know where my engine gets to it's best efficiency. 1650 Rpms for my Cummins trucks, the 7.3 liked 1800 rpm. Gearing aside, I keep my rpms there I get the best economy. My F 350 had 3.55's and a five speed, I put 35" tires on it (33.9 actual) and I if I kept the rims in the 1800-1850 range, good fuel mileage. Now, it LIKED to make power over 2k rpm so if I needed to pull a hill, pass, ect....2300+ rpm gave me the most torque to maiantian speed, but will chew through fuel.
My Cummins has 3.73's and 36" tires (35.9 actual) and I rarely use 6th in my auto when towing as 1650-1700 rpm has me at 67 mph in sixth.
Lower gears would allow me to use sixth when towing. But also, watch boost and egt's as well and you can maximize economy. You don't want to lug it and create heat and higher the boost, more fuel too.
I don't worry too much about it though, but get 11-12mpg with camper and boat in tow, YMMV.
