Speed is distance divided by time. The SI unit is metres per second (m/s), but kilometres per hour (km/h) and miles per hour (mph) dominate daily use in road transport. The knot — one nautical mile per hour (1.852 km/h exactly) — is the universal standard in aviation and maritime navigation, because it directly relates to Earth's geometry: one nautical mile equals one arcminute of latitude.
Mach number is the ratio of a body's speed to the local speed of sound. At sea level (20 °C) Mach 1 ≈ 343 m/s (1,235 km/h). At cruising altitude (−56 °C) the speed of sound drops to ≈ 295 m/s, so Mach 0.85 represents a lower absolute speed than at sea level. The ThrustSSC set the land speed record at Mach 1.016 (1,228 km/h) in 1997, making it the only car to break the sound barrier.
| 1 mph | = 1.60934 km/h |
| 1 knot | = 1.852 km/h |
| 1 m/s | = 3.6 km/h |
| Mach 1 | ≈ 1,235 km/h |
| 100 km/h | = 62.14 mph |
| Speed of light | ≈ 1.08 billion km/h |