Don't they teach Flight for Range in the U.S.?

Start at cruise RPM. Record the airspeed when things have
stabilized. Reduce the RPM by 100, wait for everything to stabilize,
including adjusting the throttle for the RPM target, trim the airplane
for level flight, and record the airspeed again. Do this in 100 RPM
increments until the airspeed suffers a much bigger drop than with the
previous settings, and go back to the last setting. Now you have the
lowest fuel flow for the best airspeed. Be sure to lean as necessary
and keep that RPM where it should be. Remember the power setting for
that airplane for the next time.
Best endurance involves finding the lowest RPM where the
airplane will maintain altitude. It'll be a pretty slow airplane. Slow
flight involves a lower airspeed but more power to maintain the
altitude.

Dan