I've got the same engine and gears with a 13spd overdrive. If mt or light, I drive in 13th gear at 1250RPM, and occasionally will split down to 12th on a hill when my RPM is pulled down below 1200, that being the bottom range of peak torque band. If the hills are constantly short rolling, I get lazy and leave it in 12th at 55mph; you have the advantage here with your automatic as it will shift back and forth perfectly every time for you. In the metro areas where the speed limit is 55mph, I will run along at 50mph in 12th. I think that 35 mph was the speed a truck drove at to achieve a record in mpg, but it's not very safe or time efficient to try and apply that style to the real world. Also, I've heard that there is no advantage to having a more aerodynamic truck at speeds below 52mph, so all else being equal, my Pete 379 vs a T660 should achieve equal fuel mileage at 51mph.