IO540, I imagine what SNS3Guppy was alluding to is that, in most light aeroplanes with constant speed propellers, propeller efficiency is higher at the lower end of the cruise RPM range (because the higher advance ratio gives a blade alpha that is closer to producing max L/D for the prop) and that this gain in efficiency is typically more significant than the gain in engine efficiency from lower friction losses and greater volumetric efficiency. However, for any particular airframe/engine/propeller combination, it is quite difficult to access data that shows the relevant contribution of each to the overall gain in efficiency that is usually achieved by flying at a lower RPM.