there are some excellent wind speed meters -use by glider guys- which operate in that speed envelope .
mounted on a plane they would show airspeed -if you flew under 30 MPH---
as for engine rpm -- on my gas engines - I have ignition systems which read and record highest rpm
These ar standard setups on most ZDZs now and some MVVS (I have both types )
these ignitions allow one to tailor prop rpm in flight
If I was into speed aircraft ( which I am not) I would match speeds with prop readings and plot a "best " hp/ prop combo.
on glow stuf you can buy some in air recording devices whch read rpm etc..