All this is why I advocate a manifold pressure gage for even fixed
pitch aircraft. It is ridiculous how little full scale error in a
tachomenter can influence such important things like fuel consumption
and engine evaluation. I've never tried the flourescent light trick at
1800 RPM. Does it work in practice?

Any audio measurements using a computer as a timebase would have to be
made in the aircraft or with the aircraft sitting still on the ground,
to eliminate sonic velocity errors. Even a musician who allows a
quarter step error to creep in would have a 2 1/2 percent error. Maybe
if he carried the tuning fork (or whatever) in the aircraft?