To measure speed with a tachymeter, time a known distance with the chronograph (stopwatch) and read the number on the scale where the stopwatch seconds hand stops. If the distance is one kilometre, the number is your average speed in km/h. If the distance is one mile, the number is your average speed in mph. For example, if one kilometre takes 45 seconds, the hand points to 80, which means 80 km/h.
What you need before you start
- A chronograph watch with a tachymeter scale printed on the bezel or dial. The scale is only useful if the watch has a working stopwatch function.
- The watch manual, or the manufacturer’s online instructions for your model. Stopwatch start, stop and reset buttons, and the hand that reads the scale, differ between calibres.
- A distance you know precisely. A marked stretch of track, a running course with a measured length, or a set of road markers whose spacing is documented. One kilometre or one mile gives a reading you can use directly.
- A safe place to take the measurement. The method is a calculation from a controlled course. Do not operate the stopwatch while driving or riding.
Step-by-step: reading the tachymeter
- Confirm which stopwatch hand reads the scale. On Seiko’s V192 caliber, Seiko’s instructions direct you to the stopwatch’s 1/5-second hand. Other calibers use a different hand, so check yours.
- Pick a start marker and an end marker that are exactly one kilometre or one mile apart.
- Press the chronograph start button as you pass the start marker. Press stop at the end marker.
- Read the scale at the position the stopwatch hand is pointing to. That number is the hourly rate for one unit of distance.
- Reset the chronograph before the next run by pressing the reset control shown in your manual. Repeat the run and average the results if you want a steadier figure.
Worked examples for kilometres and miles
Seiko’s published instructions for the V192 use a one-kilometre course. A run that takes 45 seconds puts the hand on 80, giving 80 km/h. In Seiko’s general instructions, a one-kilometre time of 72 seconds reads 50 on the scale. The table below uses the same arithmetic for several common times.
| Time for 1 km (seconds) | Scale reading | Average speed |
|---|---|---|
| 36 | 100 | 100 km/h |
| 45 | 80 | 80 km/h (Seiko V192 example) |
| 60 | 60 | 60 km/h |
| 72 | 50 | 50 km/h (Seiko general instructions example) |
The same logic works for miles. A one-mile run that takes 90 seconds puts the hand on 40, so the average is 40 mph.
Measuring a distance that is not 1 km or 1 mile
If your course is shorter or longer than one unit, multiply the scale reading by the number of units you covered:
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Scale reading × distance in km (or miles) = average speed in km/h (or mph)
Take a 500-metre course that you run in 36 seconds. The hand stops at 100 on the scale, which would be the reading for one kilometre in 36 seconds. Multiply 100 by 0.5 and you get 50 km/h. You can check that directly: 500 metres in 36 seconds is 13.9 m/s, or 50 km/h.
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The same rule works in reverse. A distance twice as long covered in the same time gives twice the reading, so keep the distance you use fixed and the arithmetic stays simple.
What the reading actually tells you
It is an average over the whole interval
The tachymeter reports the average rate across the timed distance. It does not show speed at a given moment. A run that starts slowly and finishes fast will give the same result as one that is steady at the average.
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Timing accuracy matters more than the scale
The scale is only as good as the timing. Starting the stopwatch a half-second late or stopping it early can shift the result noticeably at higher speeds. Use clearly visible markers, and press start and stop at the same point on the course each time.
The one-minute window and longer-range scales
On a one-kilometre course, the reading is only useful within about one minute. Any time over 60 seconds means your average speed is below 60 km/h, and the hand will be past the printed range for a short-range scale. Seiko’s V192 procedure specifies a measurement within one minute. Seiko’s general instructions note that some models carry scale notation for timings longer than one minute. Do not assume your watch works that way. Check the manual for the scale range on your model.
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Using the scale to measure output rate
The same method can estimate hourly output rather than speed. Time one completed item or job, then read the scale. Seiko’s instructions give the example that 20 seconds per product corresponds to 180 products per hour. This works for any repeatable task where one unit takes a consistent time.
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Common mistakes and fixes
- Reading the wrong hand. If the reading looks implausible, confirm you are following the stopwatch hand, not the running seconds or the hour and minute hands.
- Using the wrong distance unit. A kilometre course gives km/h. A mile course gives mph. Mixing them produces numbers that are off by a factor of about 1.6.
- Timing more than one minute on a short course. Use a longer course or check the scale notation for your watch.
- Forgetting to reset. Leftover elapsed time from a previous run will throw off the next reading.
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