A tachymeter scale is a scale sometimes inscribed on the bezel of an analog watch, perhaps most famously the Rolex Daytona. It can be used to compute speed based on travel time or measure distance based on speed. They are identifiable by the words “units per hour” at the beginning of the scale.
Measuring speed: If you timed how long it took to travel one mile using the chronograph, the tachymeter scale allows one to determine at a glance the speed in miles per hour by consulting the number on the bezel. The typical tachymeter scale on a watch converts between the number of seconds it takes for an event to happen and the number of times that event that will occur in one hour. The scale on a watch is only valid for things that happen in 60 seconds or fewer, and the scale is also difficult to resolve for events that take fewer than 10 seconds or so to occur.
Measuring distance: If you hold your speed constant (at, say 60mph), a tachymeter scale can be used to measure distance by timing the travel over that distance. When the second hand reaches the point on the scale where the speed indicated equals the speed of the vehicle, one unit of distance has been covered. For example, if you travel at a constant 60 mph, then the distance traveled while the second hand sweeps to “60” will be exactly 1 mile.
They also look pretty cool.