My 27 Chummy has exactly the same problem. In my case it’s because the drive mechanism runs at 3360 turns per mile, whereas the speedo head is calibrated to 2240. I keep meaning to sort it out but as long as I know 30mph = 45 there’s no urgency. The easiest, if not cheapest solution is to work out what the two calibrations actually are and get a correction box made. Speedograph richfield or similar should sort you out.
Incidentally the drive belt should be a wire spring, available from various suppliers but that’s not the cause of this problem.
Incidentally the drive belt should be a wire spring, available from various suppliers but that’s not the cause of this problem.
Alan Fairless