If your watch features a tachymeter scale it will be inscribed usually around the bezel or outer dial of the watch. It can be used to measure average speed or distance travelled. However if you want to use it to measure speed you need to be able to first measure the distance you will be travelling, and if you wish to measure distance you will first need to be able to not only know the speed you are travelling at but also hold it constant over the distance to be measured.
Watches are often sold with the imagery of pilots and fast cars, and a practical use is to measure miles or kilometers per hour but the tachymeter can actually measure the average amount of time you can do anything per hour. It uses a simple formula to convert the number of seconds for an event to happen and converts it into the number of times that would happen on average per hour:
T represents the numbers on the tachymeter’s scale, t is the time in seconds measured by the watches chronograph function for the event to happen and 3600 is the number of seconds in an hour. So if it take 30 seconds to travel one mile or kilometer or for instance in a factory to machine one part or in a printers to print one page then the scale at 30 seconds on the tachymeter gives a value of 120, meaning your average speed was 120 miles or kilometers per hour depending on which you travelled and measured, or you could expect 120 parts or pages per hour for as long as a constant speed can be maintained.
A tachymeter watch can be used to measure distance if you can measure and hold constant your speed over the distance to be measured. While travelling at a constant speed and starting the chronograph at 0 seconds, when the seconds hand reaches the number on the scale equal to your speed you have travelled one unit of which ever unit of measure you are using to gauge your speed. For instance if you travel at 60 miles or kilometers per hour it will take 1 minute to reach 60 on the tachymeter scale and so 1 minute to travel 1 mile or kilometer.