Tachymeter: How the Scale Works

What are those mysterious tachymeter scales on the bezel of so many watches actually for?

You won't find an explanation in any Rolex booklet. So here we get to the bottom of it, with the help of Matthias (Prof. Rolex) from the R-L-X forum, who kindly shared this guide:

Most people assume the bezel is a speedometer you can use to measure the speed of a car. That's one possible use, but it's very inaccurate, because the distance has to be exactly 1 km and the speed has to stay constant for the whole measurement. Those are hardly practical conditions for a measurement like that.

As the bezel says, the scale measures units per hour. Those can be kilometers, as above, but also anything else, like units produced or people transported. Basically you can measure anything you can express in units per hour (UPH).

So how does the scale work? To get from the measured seconds per unit to units per hour, you divide 3,600 seconds (= 1 hour) by the measured seconds. And that's exactly what the scale does for you.

Example

You measure 40 seconds for one unit, so U = 3600/40 = 90 units per hour. The number 90 is on the bezel at 40 seconds.

So the scale follows the math function U = 3600/x, where U is the calculated units per hour and x is the measured seconds. With the tachymeter scale you just read the value off the bezel. By the way, the function is a simple hyperbola that approaches zero asymptotically.

Another example:

In a car factory the time between two consecutive steps on the assembly line is measured at 100 seconds. On the bezel you can read that 36 cars per hour go through these steps.

One last example, to show the problem with the old Daytona bezels:

On a cable car with cabins for 6 people each, the time between 2 cabins is measured at 15 seconds. The bezel of an old Daytona, say with the reference 6263/6265, doesn't show a value at 15 seconds, because the measuring range isn't on the bezel.

That's because Daytonas built until about 1990 only have a measuring range between 200 UPH (= 18 seconds) and 50 UPH. The numbers between 60 and 50 UPH stand for 60 to 75 seconds of measuring time. That was probably a concession to using the bezel as a car speedometer, which, as mentioned above, isn't very practical. So around 1990 the Daytona bezel was changed to make it usable for things like production measurements. There you need the range between 400 and 200 UPH more than the range between 60 and 50 UPH. With a current Daytona you can easily measure the capacity of the cable car above: with the cabins 15 seconds apart, the bezel shows 240 cabins per hour, which works out to 240 x 6 (people per cabin) = 1,440 people per hour.