helps you estimate the speed you are going.
It uses the basic formula that speed = Distance/time
Here is how to use it: (For the full article see the link below)
To use tachymeter, simply start the chronograph or stopwatch
when the object measured passing the starting line. When the object reaches the next mile or kilometer marker, stop the chronograph or stopwatch. The point on the tachymeter scale
that adjacent to the second hand of the watch indicating the speed (in miles per hour or kilometers per hour) of object traveling between the two points.
For example, we want to measure the average speed of a vehicle is moving. Start the chronometer or stopwatch function when the car passes the starting line, and stop the timer after the vehicle traveled exactly 1 mile or 1 kilometer. Then look at where the chronometer hand (the second hand of the watch) is pointing to, and get the reading or value of the corresponding number at the tachymeter bezel or dial. Let's say the stopwatch stop at 3 o'clock position, meaning 15 seconds had elapsed for the car to travel 1 mile or 1 kilometer. At 3 o'clock position, the tachymetre value shows that number of 240, which means the average speed of the vehicle was 240 mph or km/h.