I (and many others) have found that lower prop RPMs deliver better MPG. I reckon 10% better at 2200 than at 2500, for example.
The reason for this is not clear but appears to be principally due to LOP combustion taking place more slowly, and a slower RPM matches the combustion pressure/time profile more optimally to the crankshaft angle.
Of course, flying slower gives you better MPG anyway, so to test this one needs to do it carefully, keeping IAS (i.e, thrust) constant, etc.
But all this is viable only with a constant-speed (variable pitch) prop. With a fixed pitch prop you may not be getting enough power to fly, at the lower RPM. And if you reduce the RPM you
will fly slower, so you
will get a better MPG, but this is not (mainly) due to the lower RPM; it is due to being closer to Vbg
