el_supremo:
If the diameter of the anemometer is D and it rotates at R revs per minute, then the wind speed is:
S = RDpi/60
The above formula will calculate the tip speed of the rotating "disc", but the wind-speed to tip-speed ratio is not 1:1.
You will need to calibrate your anemometer by plotting RPM against a known wind-speed. You could attach it to the roof top of a car, drive at varying speeds and then collect and plot RPM measurements against car speed.