Based on my initial research, I'm using a Tach reference of 2400 RPM.
But, if the plane Tach is a 2500, or 2300, or some other number, my
planning for cruise flight BHP, flight times, and costs gets all
thrown off.

Is your tach accurate? You may find that differences in the tach
=readings= swamp the fine cost issues you are trying to figure. But to
save the most money, plan for lowest RPM consistant with safe and proper
operation. The speed loss will likely be smaller than the RPM drop.

Make a graph of speed vs RPM (if it's a constant speed prop, use the
same power setting). You will probably find that percentage wise, you
get less additional speed than you put in in additional RPM.

Now headwinds should be factored in too - since changing from a GS of
120 to 100 is a smaller percent change (what you're comparing with the
RPM) than going from 100 to 80 due to a 20 knot headwind.

Jose
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