And there you have it, Jim. You're left wondering. If it's a good engine I would start with using 1500 RPM as a buffer and see how it does in the air. You should be able to hear what sounds like 1000 or 1500 RPM.
Originally Posted by
the Wasp
Lol
If you have a speed gun you can interpolate what the RPM would be from the air speed with the calculator linked above.