[Peter Serio]

You could be right at the end of it's service life. No cable driven speedometer will last for over 1/2 a century. At some point it will lock up. typically this happens when you are on the freeway. Given the RPM on the cable, for example at 60 MPH on the dial, the cable is in rotation at 1,000 RPM.

[GTOh!]

Thanks, Peter. Could/would reaching end of service life actually cause the needle to read higher by a consistent 10mph? Whether I'm traveling at 30 mph, 45 mph or another speed, it's reading 10 mph over actual. FWIW, It has been wavering (not a steady readout) on occasion ever since I got the car. I did receive a new speedo cable with the car.

Maybe the PO had an inkling that it would soon be needed? put approximately 600 miles on it since buying it last July.