What it actually does
A tachymeter scale runs around the outer edge of the dial or bezel, with numbers like 500, 250, 200, 150, 120, 100, 80, 60. To use it, you need a chronograph and a fixed distance (one mile or one kilometre). Start the chronograph as you cross the start line; stop it as you cross the finish line. The number the seconds hand is pointing at on the tachymeter scale is your average speed in mph (if your distance was a mile) or km/h (if it was a kilometre). The maths is just distance ÷ time, scaled so a 60-second course = 60 mph, 30-second course = 120 mph, 12-second course = 300 mph.
Why it exists
In the 1930s and 1940s, before consumer odometers and before motorways had mile-markers in your line of sight, race drivers and rally officials needed a way to clock cars at a known marker without doing maths in their head. A chronograph with a tachymeter was the obvious tool: you could time a car crossing a measured kilometre and read the speed straight off the bezel. By the 1960s and 70s, the tachymeter became inseparable from motorsport-coded chronographs: the Omega Speedmaster (1957), the Rolex Daytona (1963), the Heuer Carrera (1963), the Heuer Monaco (1969).
Why it is useless today
You almost certainly have GPS in your pocket. Your car has a digital speedometer accurate to 1 mph. Tracks have transponder timing accurate to 0.001 seconds. There is no everyday situation in 2026 where reading a tachymeter is faster, more accurate, or more practical than the alternatives. The scale survives because it is part of the visual identity of a chronograph: a Speedmaster without a tachymeter bezel looks wrong, a Daytona without a tachymeter looks wrong. It is dial decoration that happens to retain a mathematical function nobody uses.
How to read the scale
The scale is calibrated for any distance, not just a mile or a kilometre, as long as you keep distance and speed units consistent. If your seconds hand stops at 120 on the tachymeter after timing one mile, you averaged 120 mph. If it stops at 120 after timing one kilometre, you averaged 120 km/h. For times under 7.2 seconds the scale typically does not extend; speeds get unrealistically high (above ~500 mph), and the geometry of the bezel runs out of room. For times above 60 seconds you need a logarithmic tachymeter (used on a few specialist chronographs) or you have to halve your distance and double the answer.
Tachymeter vs other scales
A watch can carry multiple scales on the bezel or chapter ring: tachymeter (speed), telemeter (distance to an event you can see and hear, like lightning, by timing the gap between the flash and the thunderclap), pulsometer (heart rate, by timing 15 or 30 beats), and slide rule (general arithmetic, on a Breitling Navitimer). Each scale assumes a specific event-type. The Speedmaster is tachymeter-only; the Navitimer carries a slide rule plus tachymeter; older medical-themed Heuers had pulsometers. See our tachymeter wiki for the maths in detail.
Should you care?
For buying purposes: a tachymeter is part of the look. If a tachymeter bezel matches the watch you want, fine; if it does not (a Speedmaster Reduced with a tachymeter on the dial rather than the bezel), you might find the dial cluttered. Functionally, you will never use it. The same is true of most chronograph features: the chronograph itself, the tachymeter, the fly-back, the rattrapante are all tools that survived into the smartphone era as jewellery with engineering pedigree. That is most of luxury watch buying.