I was just thinking about this discrepancy and thought to myself, I wonder if an axle speedo drive off a bike with 17" or 18" front wheel would make a 19" read correct.
A 10% (high) speedo reading is normal for vintage gauges. I have mounted a Trail Tech "Endurance Speedometer" and was able to set it to less then 1/2% high. The speedo is set by the wheel circumference. This computer displays and stores lots of usefull info.
