A tachymeter is a watch scale for measuring speed, usually with chronographs.
The tachymeter scale is used for measuring the velocity of a body in movement. The measurement range is one kilometer. If you measure the speed, start the chronograph in the moment in which, for example, a vehicle passes the starting point of the measurement path and stop it as soon as it passes through the end point. Then the indication of the hand corresponds to the average speed (in km/h).