This question already has an answer here:
Given atmospheric condition, a typical common engine (say, Lycoming 4-cylinder), and a fixed-pitch normal prop:
What are normally the best altitudes for:
- ground-speed and
- fuel efficiency (range)?
Is it always highest (reasonable) altitude (say >=10,000')? the air resistance is low but so is the engine power, or is it usually some intermediate altitude?
Does it depend on the aircraft? Is a Cirrus SR-20 different from a Vans RV-9 from a Cessna 182 from a Piper Cherokee?
How does the efficiency of a piston aircraft change with altitude? covers a lot of ground about the primary forces (air resistance and (normally aspirated) piston/prop efficiency) and secondary forces. But it does not answer how these forces typically combine to create "best" altitudes (for mpg and gs). The question ultimately was about how these forces could create typical optimums at different alternatives, and whether they could be different for an SR-20 or a 182.