On a sports watch, the tachymeter is that detail that catches the eye even before the chronograph is started: a numerical scale on the bezel or on the flange, often graduated from 60 to 500 (sometimes more). Halfway between on-board instrument and aesthetic signature, it recounts a time when speed was measured by the sound of engines, the straight lines of circuits and records recorded on paper. Before GPS and applications, you needed simple, reliable and immediate tools. The tachometer was born from this culture: that of engineers, pilots, timekeepers and enthusiasts for whom a second is never “just” a second.
It is found on icons of sports watchmaking, from certain chronographs born for motor racing to pilots' watches. But beyond the legend, it has a very concrete function: calculating an average speed from a measured time. And it's much simpler than it seems.
As a child, in the playground, it was fashionable to “show off” (or “flex” as our young people say with their haircuts that make them look like alpacas) by brandishing one's wrist to show one's “tachometer” (yes, we didn't know how to pronounce it in my circle of friends)... From the interest of learning Latin and ancient Greek because we would have known, ignorant children that we were, that tachometer comes from the Greek takhýs which means “rapid” and it is therefore pronounced “takimeter”!
A tachometer does not “measure” speed live. It transforms a duration into average speed over a known distance. In the vast majority of cases, the scale is calibrated for a distance of 1 kilometer Or 1 mile. The logic is as follows:
The formula is that of school, but made elegant by watchmaking: speed = distance / time. As the scale is built on a one hour basis, the most used conversion becomes:
Speed (per hour) = 3600 / time (in seconds) for a distance of 1 unit.
The tachymeter prints this conversion directly on the bezel. Result: you no longer have to take out a calculator, just read the number which aligns with the chronograph seconds hand.
Depending on the model, the tachometer can be located:
In all cases, the function remains the same, on one condition: the watch must have a chronograph (at least one central chronograph seconds hand).
The standard tachometer is designed for one unit: 1 km Or 1 mile. The first step therefore consists of choosing a route whose distance you know precisely: a kilometer marker, a segment of track, road markers, or even a distance measured on an athletics track (by then adapting the reading, we will come back to it).
At the exact moment you pass the “zero” point (start of the kilometer), start the chronograph. On most watches, this is the upper pusher.
At the end of the kilometer (or mile), stop the chronograph. The chronograph seconds hand then indicates a time.
Look at where the chronograph seconds hand is pointing on the tachymeter scale. The number indicated corresponds to the average speed over distance traveled.
You start at the start, you stop at the next kilometer. The hand stops at 30 seconds. On a tachymeter bezel, this generally corresponds to 120. Reading is immediate: 120 km/h.
At 45 seconds the scale will give you approximately 80. SO 80 km/h average over this kilometer.
If your tachometer is rated for miles (or you are using it in that context), 36 seconds refers to 100. So you get 100mph. In practice, many watches do not specify “km/h” or “mph”: it is your reference distance which sets the unit.
The honest answer: rarely “out of necessity”. You can measure speed more easily with a phone. But watchmaking has never been reduced to utility. The tachometer remains a cultural bridge with a very precise imagination: night rallies, European circuits, cockpit instruments, the mechanical precision which transforms gesture into information.
In modern usage it may still prove relevant:
The standard tachometer assumes 1 unit. But you can use it on 0.5km, 200metc., by applying simple logic: if the distance is different, the reading must be corrected proportionally.
If you measure time on 0.5kmthe actual speed will be half of the value read (since you have only traveled half the reference distance). Example: you read 120 on the bezel after 15 seconds on 500 m; the average speed over 1 km would be 240 km/h, but your speed over 500 m corresponds to 120 km/h if you have mentally calibrated the exercise to 0.5 km. In practice, it is better to use 1 km directly when possible to avoid confusion.
A tachometer can theoretically give a cadence or speed over a short distance, but the correction becomes less intuitive. Over 100 m (0.1 km), the actual speed is a tenth of the value read if the scale is km/h. It's doable, but it's not the most “natural” use of the tool.
Why do so many watches retain a tachymeter scale, even when their owner will never use it? Because it “dresses” the watch like a white stripe on a vintage tire: it’s a code. It imposes a peripheral reading, densifies the face of the chronograph, underlines its sporting DNA. On a bezel, it brings this graphic presence which recalls the measuring instruments of another era, when elegance also involved the ability to quantify the world.
Understanding how to read a tachometer is therefore more than learning a trick: it is entering into a culture. A way of reconnecting, in the time of a kilometer, with the idea that speed is not just a number, but a story, that of a hand that runs, of an engine that revs up, and of a watch which, too, deserves to be “driven”.
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