To calibrate it get a car moving at a known velocity. Stick your anemometer out of the window and take a reading. Do that for every 5Km/h from 5Km/h to 35Km/h so you get a range of counts (pulses per second). Turn round do the same sequence of readings going in the opposite direction (that should negate any errors from it not being a flat calm day). Rinse and repeat until you get enough readings to draw a line on a graph.
You can also get an approximate average wind speed from your nearest airport or airfield. For example you can get the weather at London Heathrow with http://aviationweather.gov/forecast?id=EGLL
I can get the weather for RAF Station Odiham from http://aviationweather.gov/forecast?id=EGVO
Or from Farnborough at http://aviationweather.gov/forecast?id=EGLF