Originally Posted by
Above The Clouds
You won't reach max rpm at 7500ft that's why it's only producing 75% power with a full throttle setting provided you have leaned it correctly, the reason for it being the optimum altitude is because you will be getting the best TAS versus power and fuel consumption.
You can lean the mixture at any altitude providing you follow the engine manufactors recommendations, on some types you may even have to lean the engine during taxing and certainly if departing from high altitude airports if you don't lean the engine for take-off you won't develop take-off power for that altitude.
The first paragraph is incorrect. In aircraft with fixed pitch props the maximum RPM at full throttle stays approximately the same at any altitude. What changes is the amount of power that any given RPM produces. The higher the altitude the lower the power. The engine however is most efficient at full throttle because the throttle valve is wide open. Therefore to get full open throttle at a cruise power setting you have to fly high. One problem with this is the high RPM tends to make the airplane noisy, but otherwise your typical 4/6 cylinder non turbo charged carburated Continental/Lycoming engine is rated for unlimited operation at full RPM.
For most light aircraft the no wind sweet spot is the 6500 to 8500 altitude range at 65% power (usually around 2400 - 2500 RPM). This gives a good compromise between speed, fuel consumption and cabin noise. Fly higher with a lower power settings with a tailwind and lower with a higher power setting with a headwind.