The correct type of chart to use is a BSFC map (BSFC vs speed AND torque). I'm not sure it Cummins publically releases it's BSFC maps, but most all modern turbodiesel BSFC maps have more or less the same general shape
Click to BSFC Thread posts #170-173 on Cummins.
A chart/graph might show 1,800-rpm as optimal, but it may also NOT account for winds. Speed, as the final variant, in other words.