What function does a tachymetric scale serve on a chronograph?

A tachymetric scale is designed specifically to calculate speed based on travel time and distance. This scale is typically found on the bezel of a chronograph watch and is utilized when timing a specific event. When the chronograph is used to measure the time it takes to cover a known distance, such as a mile or kilometer, the tachymetric scale allows the wearer to easily read the average speed achieved over that distance once the time is known.

For instance, if a user starts the chronograph while traveling a distance of one kilometer and stops it after 30 seconds, they can use the tachymetric scale to determine that their speed was 120 kilometers per hour. It provides a convenient and quick way to calculate speed, which is particularly useful for motorsports and other time-sensitive activities.

Other functions present in the choices do not pertain to the purpose of a tachymetric scale. Height measurement, distance calculations unrelated to speed, and power reserve indications are not among the capabilities of this scale, thus reinforcing that the primary function relates to speed calculation over a set distance, confirming the accuracy of the selected answer.