gentlemen---thank you for your answers. let me mean "highest
continuous cruise speed." michael--I find it hard to believe that even
highest available cruise for short periods would be ground level. the
air resistance is pretty high down there.

I think most people are recommending 6000' to 8000', because it gives
75% power. is this just "experience" or "rule of thumb that I learned
somewhere" ? in other words, why is 6000-8000' where the air
resistance vs. power curves cross? or, why is 4000' not better? there
is more power. why is 10000' not better? there is less air density.

are there some rough equations that can show that 6000' to 8000' is
about optimal?

regards,

/iaw