Mileage observations

It takes a certain amount of horsepower to pull a trailer down the highway at 65 mph. In the case of the op it might be around 95 hp. When the engine is making 95 hp and running say 4000 rpm the torque it is generating is: torque = 5252 x 95/4000 = 125 lbft of torque. If the op gears up such that the engine is running 2000 rpm the required engine output is still only 95 hp but now the torque level is 250 lbft. With the engine running at 2000 rpm there is less energy lost at 2000 rpm but more significant is the fact that the higher torque means higher cylinder pressure and more efficient burning. Slowing the engine down for the same hp requirement always causes higher cylinder pressure.