The tachymetric scale doesn't measure time: it converts it. It's a fixed graduation, printed or engraved on the dial or bezel of a chronograph, that transforms a time interval in seconds into an average speed expressed in km/h or mph. The mechanism is simple and hasn't changed since the Thirties: you start the chronograph the moment you pass a reference point, stop it after exactly one kilometer (or one nautical mile), and read the value where the chronograph hand has stopped on the scale. No calculations, no conversions: the result is direct.
The scale works because it's built around the inverse of time. At 60 km/h you travel one kilometer in exactly 60 seconds, so the value 60 falls on the dial at 60 seconds. At 120 km/h it takes 30 seconds, so 120 is found at the midpoint of the scale. This explains the progressive compression of values toward the bottom: the faster you go, the fewer seconds it takes, and the numbers bunch up as they approach 15-20 seconds. A standard scale covers roughly 60 to 500 km/h, but there are exceptions: the spiral scale on the Angelus Instrument de Mesures goes down to 20 km/h, and the Tudor Pelagos FXD Chrono adopts a scale designed for cycling with minimum values lower than conventional ones.
The position of the scale varies. It can sit on the rotating or fixed bezel, on the inclined inner flange, or directly on the dial as in the Cimier 711. Engraving it on a curved bezel, as in the TAG Heuer Carrera Split-Seconds, reduces parallax error when reading the chronograph hand. On the nautical tachometer, the reference distance is the nautical mile: same logic, different unit.
Some construction details have become historical identification markers. The most famous is the Dot Over Ninety (DON): a white enamel dot positioned above the number 90 on the tachymetric scale of the original Sixties Speedmaster. Omega has reproduced it in editions faithful to the 321 caliber. Similarly, the vintage Rolex Daytona features intermediate batons and small dots next to the numerals, while the modern model uses triangles and radial numerals: same function, different legibility and taste.
From the workbench I'll add something the notes don't say explicitly: the tachymetric scale loses practical utility below 15 seconds of elapsed time, because human reaction error in pressing the button becomes proportionally significant. It was a serious tool on track in the Fifties and Sixties. Today it's mainly a marker of belonging to a sports chronographic tradition, but the physical principle remains sound.