racing props are crazy makers - More theories than you can believe---
FWIW another nifty tool I found -is the ROTO gas engine with it's own tattletale tach built into the ignition.
you shut down the plane after a flight - by throttle --then poke a optical tach in front of a LED atached to the ignition - he led is flashing at last highest rpm rate.
so you read the tach -- multiply by two (usual tach reading setup) and bingo -there is the actual unloaded prop speed!
On my 3D gasser - I prop for about 9000rpm - then look at the tach after flying to see what unloaded ( even tho this is only say under 40 mph) revs are reached . 10,000 happens easily
For a gas racer I would think this setup is of huge value.
We would just listen and guess , years back.