When writing units for speed, the time component:
Must always be the first component of the unit (i.e. seconds/metre, hours/km), since speed is derived from the formula $\text{Speed}\times\text{Distance}=\text{Time}$Speed×Distance=Time.
Can be either the first or second component of the unit.
Must always be the second component of the unit (i.e. km/hour, or m/second), since speed is calculated using the formula $\text{Speed}=\frac{\text{Distance}}{\text{Time}}$Speed=DistanceTime.
What unit is most appropriate for measuring the speed of a person while running?
What unit is most appropriate for measuring the speed of a slug?
What unit is the most appropriate for measuring the speed of a motorcyclist?