A tachymeter is simply a means of converting elapsed time (in seconds per unit) to speed (in units per hour). A tachymeter is a scale sometimes inscribed around the rim of an analogue watch with a chronograph. It can be used to calculate your current speed of motion based on the time you have been travelling.
The function performed by a tachymeter is independent of the unit of distance (e.g. miles, kilometres, meters etc.) as long as the same unit of length is used for all calculations.
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