wrote:

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" ?

You have several factors here. As you ascend, the air density decreases, so drag
decreases. A plane with the engine producing 75% power at 2000' will fly faster
than a plane producing 75% power at sea level.

The figure of 75% power is important because with most of the engines used in
light aircraft you need to use a rich fuel mixture if the engine is set to
produce more than this. That uses significantly more gas per hour.

As you ascend, the air pressure also decreases. This makes the normally
aspirated engine work harder for each slug of air/fuel mix. The pilot must keep
increasing the throttle as he climbs to get the same amount of power. At some
point, the plane will be producing 75% power at full throttle.

Many of us have assumed that by "optimal" you mean the best combination of fuel
economy and speed. That point is the highest density altitude that the aircraft
can reach and still produce 75% power.

That altitude is usually published in the operating manual for the aircraft or
otherwise available from the manufacturer. In the case of my Cessna 150, the
value was in the operations section of the owner's manual. In the case of my
Maule, I had to call the manufacturer and ask.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.