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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteA tachymeter scale turns a chronograph’s elapsed-time reading into an hourly rate for a known distance or repeated unit. Time one kilometer and read kilometers per hour; time one mile and read miles per hour. The chronograph seconds hand points to the result on the scale.
How a tachymeter works
A tachymeter is a numbered scale used with a chronograph, the watch’s stopwatch function. It relates the time taken to complete one unit to how many such units could be completed in an hour. For speed, the unit is a known distance; the scale gives an approximate average speed over that timed interval.
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It is not a live speedometer: it does not continuously show your speed or account for changes during the measurement. The chronograph seconds hand supplies the elapsed-time reading, while the tachymeter scale converts that reading into a rate.
How to read the scale for speed
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Choose a known distance of one unit, such as one kilometer or one mile.
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Start the chronograph at the beginning of that distance.
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Stop it at the end of the distance.
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Read the number on the tachymeter scale indicated by the chronograph seconds hand. The distance unit determines the result: kilometers per hour for a kilometer, or miles per hour for a mile.
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For example, if the watch times one kilometer in 30 seconds, the scale indicates 120 km/h. Casio gives this example in its official tachymeter instructions.
What if the distance is not one kilometer or one mile?
Adjust the scale reading in proportion to the distance. Casio’s instructions say to double the indicated value for two kilometers and halve it for 500 meters. The result remains an approximate average hourly speed. Use the distance unit consistently: do not interpret a kilometer-based reading as miles per hour.
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How long can you time?
A typical tachymeter scale is arranged for events completed within one chronograph-hand rotation—usually up to 60 seconds. Hamilton’s manual limits its described procedure to measurements that do not exceed 60 seconds. The usable range and layout can vary by model, so check the specific watch’s instructions if the event takes longer or the scale is unfamiliar.
Using a tachymeter for a rate other than speed
The same principle works whenever one timed event represents one unit of output. Time how long it takes to complete one job or produce one item, then read the scale as an hourly rate. Seiko’s official instructions give the example of one product in 20 seconds, which corresponds to 180 products per hour. Its instructions also cite 45 seconds for one kilometer as 80 km/h.
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Where the scale appears and what to check
The scale may be printed on the dial or placed on the bezel, and chronograph hand and subdial layouts differ. When checking a particular watch, confirm that it has a working chronograph as well as a tachymeter scale, and consult its manual for the intended range and reading method. TAG Heuer’s instructions note that tachymeter use depends on the model.
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