Tachymetre
A scale on a watch bezel or dial that, used with a chronograph, calculates average speed over a known distance.
A tachymetre (sometimes spelled tachymeter) is a scale that lets a watch with a chronograph function calculate speed. It is most commonly engraved on the bezel of a sports chronograph, with the numbers running from roughly 60 to 500.
How it works
To measure speed in units per hour: start the chronograph at the beginning of a known distance, stop it when the distance is covered, and read the value the seconds hand points to on the tachymetre scale. That is the speed in units per hour over a one-unit distance.
Covering one kilometre in 30 seconds gives a reading of 120 on the scale, equating to 120 km/h.
Where it appears
Classic tachymetre chronographs: Rolex Daytona (engraved on the bezel), Omega Speedmaster (engraved on the bezel), TAG Heuer Carrera (printed on the dial). The Daytona's tachymetre bezel and the Speedmaster's tachymetre bezel are both icons of mid-century chronograph design.
Variants
Some chronographs combine the tachymetre with other scales: pulsometre (heart rate), telemetre (distance from a sound source), or specialised diving bezels. These combined scales are largely historical.
Practical use
In practice, the tachymetre is rarely used for actual speed measurement today. It remains a design element that signals a sports chronograph heritage. Knowing it is there and what it would do is part of fluency in watch references and dial configurations.
Related: chronograph, daytona.