Check the odometer with mile posts on the highway to see if it is off. If it is off in the same direction by the same percentage as the speedometer, then change the drive gear. The change of one tooth will make a difference of about 1/19 or 6%.
If the odometer reads correct the error is in the calibration of the speedometer, which is adjustable.
If you take it out, try to turn the input with your finger tips to see how easily it turns. If it is stiff, it should be serviced, they all need service every 85 years or so.
www.marksspeedometer.com