I have just held my hand held anemometer by Davis anemometer. Davis Reading “0”mph handheld “4.5 - 2.9”mph. New battery fitted to ISS unit , so I think new Anemometer arm will have to be ordered!.When you say actual are you comparing your reading with another close by ? Height above ground and nearby buildings,trees etc can affect readings. The Met Office standard is 10m above ground on an open test site and there are correction factors if your anemometer is below this. Are you getting a low battery warning on the console ? My weather vane is moving like a good 'un at the moment, but the wind direction is showing S to SW, don't know what the sampling period is for wind.