Tachymeter

A tachymeter (German Tachymeter) is a scale on the bezel or rehaut of a chronograph used to read average speed over a fixed distance. The wearer starts the chronograph at the beginning of the distance and stops it at the end; the seconds hand then points at a value on the tachymeter scale that reads directly as speed in units per hour — typically kilometres per hour or miles per hour.

The scale dates from the early days of motor and air sport, when reliable speedometers on the vehicle were rare and the driver or pilot derived speed by timing a known stretch with a stopwatch.

How it works

The principle is a direct mathematical translation of the basic formula: speed = distance / time. If the distance is fixed at 1 kilometre (or 1 mile), speed depends only on the timed interval. The tachymeter scale is a logarithmic representation of that relationship:

• 60-second mark = 60 units/hour. One kilometre covered in 60 seconds equals 60 km/h.
• 30-second mark = 120 units/hour. In 30 seconds = 120 km/h.
• 15-second mark = 240 units/hour. In 15 seconds = 240 km/h.

Most tachymeter scales begin around 60 (corresponding to one second of measurement per minute mark) and end at 400 or 500 (just under nine seconds of measurement). Below 60 the scale is no longer mathematically unique because a measurement would take longer than a minute — the seconds hand has completed more than one revolution.

Which unit is measured

The tachymeter scale is itself unit-less. Whoever defined the distance in kilometres reads km/h. Whoever defined it in miles reads mph. Whoever defined it in nautical miles reads knots. The scale also works for other units per hour — production output in pieces per hour, pulse rate in beats per minute, provided the distance is defined accordingly.

Where it appears

Classical position: on the fixed bezel of a chronograph. The Rolex Daytona carries the scale on a separately inserted steel or ceramic bezel, the Omega Speedmaster likewise on an engraved or applied bezel. On other models the scale lies inside on the rehaut between dial and crystal (Patek Philippe 5170, some Audemars Piguet Royal Oak chronographs).

Which models carry it:

• Rolex Daytona 6263, 16520, 116520, 116500LN, 126500LN — the Daytona is effectively the tachymeter model.
• Omega Speedmaster Moonwatch since 1957 — the tachymeter scale is its constant identifier.
• Patek Philippe Ref. 5170, 5172, Nautilus Chronograph 5980 — scale on the rehaut.
• Audemars Piguet Royal Oak Chronograph 26331, Royal Oak Offshore Chronograph.
• Cartier Santos Chronograph (in some configurations).

Practical use today

In daily life the tachymeter is functionally rarely relevant. Nobody measures car speed with a stopwatch — the vehicle or smartphone speed system delivers the data instantly. The scale remains a design signature of the chronograph tradition, as established on sports references as the cyclops on the Datejust.

At our atelier in Munich we nonetheless see the tachymeter actively used: customers time lap intervals on the racetrack (Hockenheim, Nürburgring), some use the scale to determine hourly output of recurring work steps. It is also useful for distance measurement in yachting.

Service and authenticity

On Daytona models the tachymeter bezel is a central authenticity marker. Font, colour treatment of the scale (steel relief engraving, white-filled Cerachrom scale) and the relationship of numerals to hour markers follow strict manufacture conventions. A replaced or re-stamped bezel on a vintage Daytona reduces value; on current Cerachrom Daytonas we change bezels only via the certified Rolex workshop.

On Omega Speedmaster models the bezel inscription varies historically — early references carry "Base 1000", later "Tachymètre" in French or English. Typographic detail helps narrow down the production year.