We’ve not done any official surveys, but we suspect that most owners of chronograph watches with a tachymeter scale on the bezel (or dial) mostly appreciate the latter for giving their timepieces a more tool-ish look. But here’s what it’s actually for: The measurement of average speed over a known distance, or distance travelled based on a known speed.
To use the tachymeter, the chronograph is started when the moving object starts from the starting point of the known distance, 1km for instance. When the object passes the finishing line, having travelled 1km, the chronograph is stopped.
Using the ceramic bezel of the latest Rolex Daytona, an object that takes nine seconds to travel one kilometer is travelling at 400km/h. On the same tachymetric scale, an object that takes 55 seconds to travel 1km is going 65km/h. The same holds for an object travelling one mile, since the tachymeter scale is units-agnostic. So now you know.