A tachymeter converts a timed interval into a rate per hour. On a conventional chronograph, you read the scale against the stopped stopwatch seconds hand. It does not measure distance or continuously show your current speed. You supply the known distance or task; the watch supplies the elapsed time.
Identify the scale and timing hand
Find the tachymeter on the bezel or around the dial, then identify the hand that measures stopwatch seconds. Do not read the ordinary running-seconds hand by mistake. Our chronograph reading guide separates the displays and explains the basic timing sequence.
Check the actual model’s start, stop and reset instructions. Begin from the specified zero position and stop the measurement before reading it. Do not assume a tachymeter bezel rotates like an elapsed-minutes bezel; its printed numbers perform a different job.
Time one known distance
TAG Heuer’s tachymeter guide describes timing travel over one mile or one kilometre. Start at the beginning of that distance and stop at its end. The scale then gives the average speed in the corresponding distance unit per hour.
For example, one kilometre covered in 40 seconds gives 90 km/h: 3,600 seconds per hour divided by 40. One mile in the same time gives 90 mph. The scale has not converted miles into kilometres; the measured distance determines the unit. These are illustrative calculations, not road-test results.
Apply the same idea to one task
A rate can also describe repeated work. Seiko’s 8R46/8R48 instructions explain timing one completed job and reading its equivalent hourly rate. A task lasting 30 seconds corresponds to 120 tasks per hour if that pace continues without interruptions.
That is a rate extrapolated from one interval, not proof that 120 tasks were completed. Setup time, pauses and changing conditions affect actual output. Define the beginning and end consistently; otherwise two measurements may describe different amounts of work.
Respect the printed range
Seiko’s illustrated method uses an interval within one minute. Check both ends of your own scale: its highest printed rate also limits the shortest directly readable interval. If timing continues into another revolution, the seconds hand alone no longer represents the full elapsed time.
Keep the total interval and calculate the rate separately when the scale cannot give a direct reading. Small markings and reflections can also make a result hard to judge; see our dial-legibility guide. Explore related everyday watch topics in Watches & Wear.