Every engine is different .. what you need to see is a chart/graph of BMEP vs. Fuel Burn to determine the engines most efficient RPM ... with a load equal to whatever “road horsepower” (rolling resistance plus aerodynamic drag) you need for a certain speed.. BMEP alone is useless on a dyno if you are just looking for max horsepower.
Generally speaking, the smaller engine operating at closer to peak output is more efficient. If this weren’t true, hybrid cars would not get better mileage than non-hybrids.
Is it where HP/torque cross and BSFC is the lowest or a point where volumetric efficiency has peaked? Yes airplanes engines are different...