tachymeter
/təˈkɪmɪtər/
noun
- A scale on a watch or chronograph used to measure speed based on time over a fixed distance.
- The tachymeter bezel is a common feature on pilot and racing watches.
- He used the tachymeter on his watch to calculate the car's speed over a mile.
- She timed the runner with a stopwatch that had a tachymeter scale.