Physics says that a normally aspirated engine will make 75% power up to 8000 ft ( assuming ISA ) so as long as the density altitude calculation is done you can pick the best altitude or flight level for the cruise.
As the temperature rises the TAS increases ( ref ISA ) so in general the higher you go the better TAS you get so it becomes a balance between wind speed & direction and TAS to get the optimal cruise altitude.
The final part of cruise altitude selection is turbulence, an aircraft that is fighting turbulence with lots of flying control inputs will be less efficient than one cruising in smooth air and this factor may result in the selection of cruise altitude above the theoretical best cruise altitude for airframe efficiency and passenger comfort.
Last edited by A and C; 3rd Mar 2020 at 15:08.
Reason: The curse of predictive text