How a tachymeter works

A tachymeter scale converts elapsed seconds into a rate per hour. It is printed on the bezel or dial of many chronographs, and it is read with the chronograph seconds hand.

Measuring speed

1. 1Start the chronograph as you pass a known distance marker — say, a kilometer or mile post.
2. 2Stop it as you pass the next marker.
3. 3Read the tachymeter number where the chronograph seconds hand stopped. That is your speed in kilometers or miles per hour.

Under the hood it is simply 3600 divided by the elapsed seconds. 20 seconds for a kilometer means 3600 ÷ 20 = 180 km/h.

It is not just for speed

Time how long it takes to produce one of anything — a coffee, a widget, a lap — and the tachymeter tells you how many you could make per hour.