If you get a speeding ticket sometimes having the speedometer calibration done will reduce the charge.
Speedometer are often off by at least a few miles per hour.
The reasons vary. Changing the size of your tires can make the speedometer off for example. Even new cars from the factory can be off. I know a calibration shop owner who tested his own new car a it was off by four miles per hour. It is good to know in general.
Speeding Tickets are based on the officer's calibration.
The officer must have his own calibration to testify to your speed. That can be based on a pace of your vehicle and him observing his speedometer. It can be based on radar or increasingly with lazars.
A valid approved calibration can reduce your ticket.
If you show the court your speedometer gave you wrong information often the judge will reduce your ticket by the amount the calibration shows it was off. It must be from a reputable calibration shop or the court will not accept it. And sometime the calibration will go against you such as saying at 75 mph the speedometer says 80 mph...in that case do not try to say the speedometer was to blame.