Originally Posted by 24Carrot
There is a different question that I think can be answered. With optimum power settings you get better miles per gallon at altitude (you can see it in e.g. the C172 and R22 performance charts) and I believe that is because of the more efficient blow cycle.
My bet would be on reducing pumping losses, since at "optimum" altitudes (6000ft and above for normal aspirated engines) you are flying WOT (wide open throttle) instead of closing the throttle at lower altitudes to remain RPM below the 75% of rated power, where you can lean to peak EGT, which in turn gives you somewhat best economy.