Convert mach (speed of sound) to minute per kilometer

Runners think in pace, minutes per kilometre or per mile, which is the reciprocal of speed: 5:00 min/km is 12 km/h, 6:00 min/km is 10 km/h, and 8:00 min/mile is 12.1 km/h. A 4-hour marathon needs 5:41 min/km, a 25-minute 5 km 5:00 min/km. Swimmers use seconds per 100 m; treadmills usually display km/h or mph, so the conversion comes up on every gym visit. Note that pace is the only unit here where a smaller number means faster.

Mach 1 is the speed of sound, which depends on the air's temperature rather than its pressure: 340 m/s (1,225 km/h) at 15 °C at sea level, the value used here, but only 295 m/s (1,062 km/h) in the −57 °C air at airliner cruising altitude, so a Mach number is not a fixed speed. Airliners cruise at Mach 0.8–0.85, Concorde flew at Mach 2. The speed of light in vacuum is exactly 299,792,458 m/s, since the metre is defined from it; light takes 1.3 s to reach the Moon and 8.3 minutes to arrive from the Sun.