How to use a tachymeter
Short answer: A tachymeter turns a time of 60 seconds or less into a rate per hour. Start the chronograph as you pass one marker, stop it at the next, and read the number the chronograph seconds hand points to on the scale. Over one kilometre that number is your speed in km/h; over one mile it is mph. The maths behind it is 3600 divided by the seconds.
What the numbers on the bezel mean
The ring of numbers running from 500 down to 60 on a Speedmaster or MoonSwatch bezel is a tachymeter scale. Many people call it a tachometer, which is strictly the name for a car's rev counter. It doesn't measure anything by itself. It's a printed lookup table for one sum: units per hour = 3600 ÷ seconds, because an hour has 3,600 seconds.
Each number sits at the second mark where that sum comes out right. 120 sits at 30 seconds, because something that takes 30 seconds happens 120 times an hour. 90 sits at 40 seconds, 80 at 45 and 60 at the very top, at 60 seconds. The unit is whatever you timed: kilometres give km/h, miles give mph, nautical miles give knots, and parts off a production line give parts per hour.
Why it starts at 500 and stops at 60
The scale has to fit inside one lap of the chronograph seconds hand. At the end of the lap, 60 seconds, the answer is 60, and after that the hand starts a second lap over the same numbers. At the other end, the numbers climb faster and faster as the time gets shorter: 7.2 seconds gives 500, 6 seconds gives 600, 3.6 seconds gives 1,000. Squeezing all of that into the last few degrees before 12 o'clock would make the marks unreadable, so most makers stop at 500, level with 7.2 seconds. Some start lower, at 400 (9 seconds).
How to use a tachymeter, step by step
- Pick two markers a known distance apart. Kilometre or mile posts, the lines on a running track, or a measured stretch of road you know.
- Reset the chronograph. The central seconds hand must start from 12, because that is where the scale starts. If it doesn't sit on 12 when reset, fix that first (see the links below).
- Start it as you pass the first marker. Press the top pusher.
- Stop it as you pass the second. Press the top pusher again.
- Read the scale where the chronograph seconds hand points. Not the running seconds in the sub-dial, and not the minute counter. If the hand is between two marks, estimate.
Worked examples
- Speed over one kilometre. The hand stops at 25 seconds and points just past 150, at 144. You were doing 144 km/h. A gentler one: 40 seconds reads 90 km/h.
- Speed over one mile. 48 seconds between two mile posts reads 75: 75 mph. Same scale, different unit.
- Items per hour. A barista finishes a coffee every 45 seconds, so the hand stops at 80: 80 coffees an hour if they keep that pace. For quick, small items, time a batch instead: 10 parts in 36 seconds reads 100, and 100 × 10 = 1,000 parts an hour.
When it takes longer than 60 seconds
Once the hand passes 12 a second time, the scale gives a wrong answer that looks right. 1 km in 75 seconds leaves the hand at 15 seconds, pointing at 240. The real speed is 48 km/h. You have three ways round it:
- Time a shorter distance. Use half a kilometre, then multiply the reading by the distance. A cyclist covers 0.5 km in 50 seconds: the scale reads 72, and 72 × 0.5 = 36 km/h.
- Do the sum. 3600 ÷ 75 = 48. For a slow job the same works: one item every 90 seconds is 3600 ÷ 90 = 40 an hour.
- For fast things, time a longer stretch. At high speed a kilometre goes by in a few seconds, on the crowded top of the scale, or before the scale even begins if it takes under 7.2 seconds. Time 2 km instead and multiply: a train covering 2 km in 30 seconds reads 120, and 120 × 2 = 240 km/h.
The rule behind all of these: rate = reading × distance timed, with the distance in the unit you want the answer in.
Try it: tachymeter calculator
How accurate is it?
The scale is exact. The weak points are your thumb and your eyes.
- Reaction time. Starting and stopping by hand adds roughly 0.2 seconds of error at each end. On a 40-second timing that barely matters: 39.8 to 40.2 seconds reads 89.6 to 90.5. On a 7.2-second timing the same error gives anything from about 486 to 514.
- Crowded marks near the top. Between 500 and 200 the numbers are far apart in value but close together on the bezel, so you are guessing between marks that may be 50 apart. Readings in the 60 to 150 range, which means timings of about 24 to 60 seconds, are the most useful.
- Distance. The answer is only as good as your markers. Roadside posts can be a few metres out, and hitting a marker exactly at speed is harder than it sounds.
- It gives an average. If you sped up or slowed down between the markers, the reading is the average speed over the stretch, not your top speed.
Fixed bezel or rotating bezel
On the Speedmaster and the MoonSwatch the scale is fixed, so its starting point is always at 12 and it only works with a chronograph reset to zero. A few watches put the tachymeter on a rotating bezel or an inner ring. Turning it lets you line the start of the scale up with wherever a running seconds hand happens to be, so you can time without a chronograph. The catch is that you must remember to line it up every time, and a bezel left out of position gives wrong readings.
Which watches on this site have a tachymeter
The Pink and Super Blue Moonphase MoonSwatches are the exceptions. Their bezel is a pulsometer, for heart rate, so the numbers mean something else entirely.
Every reading depends on a chronograph that starts from 12. If yours doesn't, see how to reset chronograph hands to zero, or the MoonSwatch chronograph reset for the pusher sequence.
Tachymeter questions
What is the difference between a tachymeter, a telemeter and a pulsometer?
All three are printed scales read against the chronograph seconds hand. A tachymeter gives a rate per hour (speed, output). A telemeter gives distance from the speed of sound: start at a lightning flash, stop at the thunder, and read kilometres; sound covers about 1 km in 3 seconds. A pulsometer gives heart rate: start, count the number of beats printed on the scale (often 15 or 30), stop, and read beats per minute.
Can I use a tachymeter without the chronograph?
Only on a watch whose central seconds hand runs all the time. Start counting when it passes 12 and read the scale when the event ends. On the Speedmaster and MoonSwatch the central hand only moves when the chronograph runs, and the running seconds sit in a sub-dial, so you need the chronograph.
Is it a tachymeter or a tachometer?
On a watch it's a tachymeter: "tachy" for speed, "meter" for measure. A tachometer is the rev counter in a car. Plenty of people call the watch scale a tachometer, and they mean the same thing.
Does the tachymeter work in mph as well as km/h?
Yes. The scale has no units. Time one mile and the reading is mph; time one kilometre and it's km/h.
Why does my reading jump around so much at high speeds?
Because short timings sit on the crowded part of the scale, where a tenth of a second moves the answer by several units. Time a longer distance and multiply, as in the 2 km example above.