For miles per hour and kilometers per hour, using the digits followed by mph and kph, respectively, is acceptable in all cases: a 98 mph pitch; the car reached 360 kph

Likewise, bpm and rpm are acceptable in all references in copy to “beats per minute” and “revolutions per minute,” respectively.

All other units of speed or rate should be written using digits, followed by the unit of measurement written out: 450 feet per second; 2 inches per year