I gave up on trying to calibrate 60 year old speedometers. Sure, it is possible to put on an adapter to make it read 50 MPH when you are actually going 50, but will it read 25 MPH when your actually going 25? There is a 60 year old hair spring, and dirt and dried up lube in there that has done some wear damage.
A brand new speedometer should read correctly throughout the entire range, but are the brand new ones built well enough to read accurately from 0-70 MPH? When you pay someone to calibrate a speedometer, their machine checks the entire range.
I drive mine like I stole it and still can't buy a speeding ticket. The only time I get pulled over by the police is so they can drool on it.
