我簡單的說，開啟秒表計時，當經過壹公裏的路程時，秒針所指的那個數字（如120）就是妳剛才的時速——120公裏每小時。
What is a tachymeter?
The tachymeter is used in the stopwatch mode to measure speed. Two things are needed to be able to judge this. First, the person must be traveling 60 miles or kilometers per hour. Second, there must be a way to judge how far a mile or kilometer is. Mile markers on a highway are a good example.
Start the stopwatch when you pass a mile marker. The seconds hand starts moving, and as it is going around it is pointing at the numbers on the bezel. If you were to pass the next mile marker and stop the stopwatch, the number that is being pointed at would be the speed that you are traveling.