A Tachymeter, is a numerical scale either printed around the edge of the dial or engraved into the bezel of a wrist watch. A tachymeter lets the wearer measure the average speed over a known distance, i.e the speed at which the watch’s wearer travels over a fixed period of time. The scale helps the wearer convert elapsed time (in seconds per unit) to speed (in units per hour), most tachymeters can make measurements from 7 seconds to 60 seconds. Tachymeter scale value = 3600 (number of seconds in an hour) divided by the elapsed time in seconds.
Tachymeters usually are found on chronographs, a chronograph is a type of wrist watch that can be used as a stopwatch or timer. To use the Tachymeter the wearer needs to know the distance between two points, as the wearer passes point one the chronograph is started, after crossing the second point the chronograph is stopped. The wearer then observes the second hand of the chronograph on the minute scale of the wrist watch, the corresponding value on the Tachymeter scale is the speed.
Tachymeters are most commonly used by athletic coaches to train their athletes on a track. Since the track, has a fixed distance, the coach is able to measure the speed of their athlete.