Best fuel economy that I've ever gotten out of our 6BT Cummins was to set the throttle setting on flat and level to put the engine in it's sweet spot (1750 rpm), and then HOLD that pedal setting. Let the engine and trans do whatever they were going to do to get us over I-5's Grapevine Pass.
18.9mpg over ~300 miles unlifted on metric 35's with a pop-top camper in the bed. Dang, but that was boring! And difficult, my subconscious REALLY wanted to change that throttle setting. I had to be very vigilant to not do that or the whole experiment was a loss.
I suspect that most don't realize how much their foot changes the pedal setting in the normal course of driving.