tachymeter

A tachymeter is a scale on some chronograph watches that allows the wearer to measure speed over a known distance or to compute production rates from elapsed time. The scale is usually engraved around the dial rim or on the bezel and is read with the chronograph function.

To use it, start the chronograph at the beginning of the distance and stop it at the end. The elapsed time in seconds is translated by the tachymeter into a speed reading in units per hour. For example, a 1-kilometer distance timed in 36 seconds corresponds to about 100 km/h.

The tachymeter is typically calibrated for a fixed distance and assumes constant speed over that distance. The scale is logarithmic, and common readings cover roughly 60 to several hundred units per hour. Some watches provide options for miles per hour or kilometers per hour depending on the metric used.

Historically, tachymeter scales emerged with the development of portable chronographs in the 20th century and became associated with racing and aviation. They remain a signature feature of many racing chronographs from manufacturers such as Heuer, Omega, and Rolex.

Limitations include the need for an accurate, fixed distance and precise timing; the scale yields a rate only for that distance and is not meaningful if the distance or path changes. It is not a substitute for a true speedometer.