Speed is distance divided by time. The SI unit is meters per second (m/s), road speeds are in kilometers per hour (km/h) in most countries and miles per hour (mph) in the US and UK, and 1 m/s is exactly 3.6 km/h. A knot, used by ships and aircraft, is one nautical mile per hour, or 1.852 km/h.
Kilometers per Hour (kph or km/h)
Kilometers per hour (kph or km/h) is the number of kilometers covered in one hour. Road speed limits in most countries are given in it.
One kilometer per hour is equivalent to approximately 0.621 miles per hour (mph).
To convert kilometers per hour to miles per hour, multiply by 0.621 – 100km/h is about 62.1mph. Multiplying by 0.6 is a rough guide that comes out a few percent low.
Miles per Hour (mph)
Miles per hour (mph) is the number of miles covered in one hour. Road speed limits in the United States and the United Kingdom are in it, along with a few other countries, while aircraft use knots.
To convert mph to kph, multiply by 1.60934 – 60mph is 96.56km/h. Multiplying by 1.6 is close enough for a rough figure.
Meters per second (m/s)
Meters per second (m/s) is the SI unit of speed. 1 m/s is exactly 3.6 kilometers per hour, or about 2.24 miles per hour.
The speed of sound in air is about 343 meters per second at 20°C, while a cheetah can reach about 29 meters per second (104 kilometers per hour).
Mach
Mach is speed relative to the speed of sound, so Mach 1 is about 343 meters per second (1,235 kilometers per hour) in dry air at 20°C. It is named after the Austrian physicist Ernst Mach. The speed of sound falls with temperature, so Mach 1 is slower at altitude.
Speeds below Mach 1 are subsonic, above it supersonic and above Mach 5 hypersonic, so an aircraft flying at Mach 2 is travelling at twice the speed of sound.