Every tug and tow has a sweet spot for best mileage. And, I believe it's doubtful any two are exactly alike. Factors such as elevation, highway grade, tires
and even the kind of oil used in the tug affect the mpg. It doesn't take too many trips to figure out the best speed for someone to tow, if the highest mpg is desired.
My combination in mostly flat freeway travel is 57 mph. And the fuel mileage can drop drastically with every 5 mph increase in speed. This was huge, when gas was running over $4 a gallon.