A tachymeter is a scale on a watch that measures speed based on how long you travel over a fixed distance. It's usually printed around the bezel or the outer edge of the dial, and you read it using the chronograph seconds hand. Time a mile or a kilometer, and the scale tells you your average speed.
How a tachymeter works
Start the chronograph as you pass a known point. Stop it after exactly one mile (or one kilometer). Wherever the seconds hand lands on the tachymeter scale is your average speed in units per hour. Cover a mile in 30 seconds and the hand points to 120 — you were doing 120 mph. The scale only works for events under 60 seconds, because it's calibrated to a single revolution of the hand.
What else you can measure
It isn't just for speed. Because the scale really measures units per hour, you can flip it to measure a production rate or anything happening at a steady pace. Time how long it takes to make one part, and the scale tells you how many you'd finish in an hour.
Do you actually need one?
Honestly, most people never use it. The tachymeter earned its place on racing chronographs decades ago and stuck around as a design signature — think of the classic motorsport chronograph look. Treat it as style first, function second. And remember it's useless without a chronograph, since the scale does nothing without a stopwatch hand to point across it.