A tachometer, also known as a tachymeter, is an instrument on many sophisticated watches that lets you determine the speed at which you, or another person or object, are traveling. If your watch has a tachometer, it will have a ring of numbers encircling the outside of its face, usually running from 500 to 60. It will also have a chronograph, or stopwatch, function, since you need this feature to use the tachometer. Tachometers are only present on analog watches.

Press the "mode" button on your watch until you are in chronograph mode. At this point, the second hand on your watch should stop moving when it reaches 12 o'clock.

Pick a point spatially ahead of you where you would like to start your speed measurement. For your first practice, use a mile marker while driving. As you pass this marker, push the set button to start the chronograph. The second hand will proceed clockwise around the face. Press the set button again when you have traveled one mile. At this point you will pass a second mile marker.

Look at the second hand of your watch. Determine what number position it is pointing to on the tachometer dial. This number represents the speed at which you were traveling in miles per hour. For example, if you took a minute to drive the mile, it will point to 60, meaning you drove at a speed of 60 miles per hour.

Measure things that move slower than 60 miles per hour by taking the tachometer’s reading and dividing it. For example, there is no way a runner can travel one mile in a minute. Instead, record the amount of time it takes him to go a preset fraction of a mile. For example, measure one eighth of a mile, then divide the result on the tachometer (say, 60, if he completed the mile in one minute) by eight. You will get the result that he runs a 7.5-minute mile.

Do the same for objects that are too fast for the tachometer, except multiply your results instead of dividing them. The smallest increment it can record traditionally is 7.5 seconds.