<VV> slow speedo

[ncoonen at fastbears.com]

I have a '61 Lakewood three-speed with 185/80-13 tires on the original steelies. The speedometer reads low across the scale. If the speedo reads 60, the car is going 70+. The needle doesn't skip or jump as I would expect with a failing cable. Any clues? Is the speedo head in the dash just getting old? The car just turned 70k miles and is quite original, right done to the worn out front bench seat. 

Thanks...ned.