Hi Davie
Airhead speedo's like most others are reckoned to be optimistic except the North American 85 MPH version which is supposed to be close to spot on.
Truth is I have never checked mine other than to observe RPM vs MPH at different speeds according to the chart below. On that basis and if the rev counter is accurate (a big IF perhaps) my speedo is not too bad reading 5 mph high at 60 MPH.
Airhead speedos are matched to the final drive ratio fitted to the bike so you might want to check you have the correct one fitted. On the lower part of the speedo face you should see W1441 which is correct for a 3.89:1 final drive ratio. I read somewhere that the W means 'Wegsteckenzahl' ('travel-distance-index-number'), which equals the number of rotations for 1 mile.
BTW I calculated that if you multiply your final drive ratio by 370 it should give you the correct speedo ratio. Can anyone check their speedo and confirm if I am close with that figure.
