A tachymeter is a function in a watch that allows one to measure average speed, given that the distance covered is known. It is usually found in sports watches and comes in great handy during a race.
A tachymeter is quite simple to use. Simply begin the chronograph at the starting marker of a known distance that is to be covered. At the next marker, stop the chronograph, and the value on the tachymeter scale at which the chronograph hand points is the average speed of travel, in distance covered per hour. A standard tachymeter scale works on the following formula:
Where S is the average speed as displayed on the tachymeter scale, t is the time in seconds counted by the chronograph and 3600 is the number of seconds in one hour. The purpose of the ‘3600’ value in the formula is to display speed in kilometres/hour or miles/hour, which are the standard units of measuring speed, as opposed to kilometres/second or miles/second.
If you are covering a distance of say, 3 kilometres, start the chronograph at the starting point and stop it as you cross the finish line. Now the value on the tachymeter scale at which the chronograph hand points is the average speed for 3 kilometres. If, for example, the value on the tachymeter scale is 150, then simply divide this by 3 (the distance in kilometres you have covered), to get the speed in kilometres/hour. Which, in this case, would 150/3=50. So, in this manner, you will get to know that your average speed of travel was 50 km/h.
A tachymeter is specifically useful during races. While a speedometer in your car will tell you the exact speed at a particular moment, it will not tell you the average speed of travel over a certain distance. For this purpose, the tachymeter on a watch is extremely useful. Of course, tachymeters have a certain weakness that they can only calculate average speed for a distance covered within a minute as the chronograph hand goes back to the base reading of the tachymeter after one circle of rotation.
Tachymeters are available in a number of sports watches. Some of the most popular of these timepieces include the the Rolex Daytona, Omega Speedmaster and the TAG Heuer Carrera among others. Most racing-inspired watches are, in fact, characterised by the presence of a tachymeter and a chronograph complication. These two are the most essential functions that define a racing watch.
In spite of the fact that they are a little bit complicated in terms of usage, especially in cases where distances are not in whole numbers, tachymeters are greatly useful in calculating average speed. In a high-speed racing event, where every spilt-second matters, knowing the average speed over a given distance can sometimes make all the difference.