Start the chronograph or stopwatch when the person or object has crossed the starting line or left the starting point.
Stop the chronograph or stopwatch when the object has passed a second point marking a fixed, easily determined distance from the starting point.
As an example, say that you've stopped the chronograph or stopwatch after the object traveled exactly 1 mile from start to finish.
Note where the chronograph hand, which is the second hand on your watch, has stopped. Determine which number on the chronograph or stopwatch bezel the second hand is pointing to and which number on the tachymeter bezel the second hand is pointing to. The number that the second hand is pointing to on the tachymeter bezel is your speed reading.
Again, for the sake of example, say that upon stopping your chronograph or stopwatch, the second hand wound up pointing to the number 5 on your watch. This corresponds to the number 25 on your chronograph or stopwatch bezel, signifying an elapsed time of 25 seconds. This will in turn correspond to the number 140 on your tachymeter bezel, signifying a speed of 140 miles per hour.
Adjust readings based on whether the object you're measuring is very slow or extremely fast, as all tachymeter readings must fall within a set time interval of 7.2 to 60 seconds.
If you're measuring the speed of a slow-moving object, you may need to shorten the distance between the start and finish points to get a reading within the 7.2 to 60 second range. For example, if you shorten the distance the object travels from 1 mile to 1/4 mile and the second hand stopped on a tachymeter reading of 90, then the object's speed is 90/4 = 22.5 miles per hour.
If you're measuring an extremely fast object, you may need to multiply the results. For example, if the object traveled 3 miles in only 15 seconds, the second hand would give a reading of 240. Because the object traveled 3 miles instead of 1 mile in that short time, you would multiply the tachymeter reading of 240 miles per hour by 3 to get 720 miles per hour.