The most common use of a tachymeter is for measuring the approximate speed of a vehicle over a known distance.
e.g.) Based on how many seconds it takes a vehicle to travel 1km or 1 mile (the available measuring range is up to 60 seconds), the average speed within the distance can be calculated.

1) Start the chronograph when the vehicles commence travel.
2) After the vehicle has travelled 1 km/1mile, stop the chronograph.
The approximate average speed within the distance can be determined by observing the present position of the second hand and reading the outer bezel.

Note: The tachymeter indications may appear on dial ring, rather than on the outer bezel (depending on model).

As shown in the illustration, it takes the vehicle 45 seconds to travel 1km so the approximate average speed is 80kph (50 mph).