Did you notice that by dropping from 6th to 5th and slowing down from 65 mph to 55 mph he said his engine rpm increased from 1401 to something like 1565. Cummins says slowing down should have gained him 1 mpg for the lower wind drag and Shiner says he should gain some fuel economy speeding his engine up a bit. Combined you’d expect a fairly significant gain. Turns out it was only a gain of .4 mpg. I thought the Cummins rock solid rule of gaining 1mpg sounded reasonable but my take away would be that the gear drop hurt a bit so the net gain was only .4 mpg.
