tachymeters

A tachymeter is a scale found on some chronograph watches and other instruments that converts elapsed time into a speed or rate. The term derives from tachy, meaning fast, and metron, meaning measure. In practice, a tachymeter provides a quick way to compute units per hour when the distance traveled is known.

To use a tachymeter, start the chronograph at the beginning of a measured distance (commonly 1 kilometer or 1 mile) and stop it at the end. The seconds hand will point to a value on the tachymeter scale, indicating speed in units per hour. Since speed equals distance divided by time, the scale is essentially 3600 divided by the elapsed seconds for a fixed distance. For example, covering 1 km in 25 seconds yields 3600/25 = 144, so about 144 km/h. The scale is calibrated for a fixed reference distance and is most meaningful when speed is relatively constant within the usable range.

Tachymeter scales are typically printed on the bezel or dial of mechanical chronographs and are common in racing- or aviation-inspired watches. They can also be used to estimate production rates by timing how long it takes to complete a fixed number of units. Limitations include reliance on precise timing, the assumption of uniform speed over the measured distance, and a finite usable range (often around 7.2 to 60 seconds, corresponding roughly to 500 to 60 units per hour).

Terminology-wise, tachymeter is the specific speed-scale feature on a chronograph; tachometer more broadly refers to devices that measure rotational speed.