So here’s what the cop told me as he was writing my speeding ticket. (Of course I told him my Speedo must be way off.) …..

When driving down a highway that has mile markers on the side of of the road, hold a steady speed. Time how many seconds it takes to travel from one mile marker to the next. Divide 3600 by the number of seconds it took to travel the mile. The resulting number is your speed in MPH.

Example: 3600/56 seconds = 64.29 MPH.

About recalibrating the cylinder type speedometers….
Once you’ve determined how far off the “ribbon” is from your actual MPH, take the speedo apart.
You will now see how it works - the cable spins a circular magnet inside one end of the cylinder.
There is a sort of clock spring on the opposite end of the cylinder that provides resistance to the turning cylinder.
Note how many degrees you have to roll the cylinder in order to make the ribbon gain or lose the appropriate amount.
Adjust the clock spring the same number of degrees, being cognizant of whether it needs to provide MORE resistance (Speedo was reading too high) or LESS resistance (Speedo was reading too low).