看了这个卡西欧官方网站对计速器（TACHYMETER）的说明，你就知道了。
我简单的说，开启秒表计时，当经过一公里的路程时，秒针所指的那个数字（如120）就是你刚才的时速——120公里每小时。
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.
我已经回答过了。
http://zhidao.baidu.com/question/80676638.html