An anemometer measures the speed of the wind. There are a few types. For this, we're concerned with cup type anemometers.

After a little thought about how one works, it seems to me that in a perfect world, the speed of circle the cups trace out, the "circumference" would match the speed of the wind.

However, factors like bearing friction, aerodynamic friction of the cups moving through the air, rotational inertia, any measurement induce loss, etc will slow the cups down.

For instance, a Davis 7911 anemometer cup set has a radius of 3" from the shaft center to the outside of a cup. The circumference is then 3.1416 * 3 * 2 (pi x the diameter) or 18.85" inches, or 1.57 feet.

A mile is 5,280 feet. That would be 3,361 rotations. So, in the perfect world, a one MPH wind would cause 3,361 rotations of the cups.

However, in their data sheet for this device, Davis says that 1 MPH = 1,600 rotations in one hour. That's a correction of 2.1, or 47.6%.

I'm certain that the correction factor will be different for different designs of cup assemblies.