You could send the speedo out and get it cleaned and replace any worn parts. The speedo IS 53 years old and is bound to have worn parts. I always have it calibrated it as well. My 65 FSM gives the pinion size by color/number of teeth/transmission/rear end gearing for the OEM tire size. You will have to convert the current sizing of your tires to the OEM size. Pull the gear and see if it is correct or you need to change it based on your tire size and gearing.
I have a few of different size pinion gears if needed.
