Speed units explained
Speed is a distance divided by a time, so every unit here is one length unit paired with one time unit. The interesting one is the knot, which is tied to the shape of the Earth rather than to anything human.
Metre per second m/s
Base unit — every factor on this page is measured against it
The SI unit, and the one physics is written in. Rarely used for vehicles because the numbers come out unhelpfully small — a car on a motorway is doing about 30 m/s. Multiply by 3.6 for km/h, which is the most useful conversion in this category.
Kilometre per hour km/h
1 km/h = 0.27777778 m/s Exact
The road speed unit almost everywhere. Sized so motorway limits land on round numbers: 100, 120, 130. A useful anchor is that 100 km/h covers a kilometre every 36 seconds, and a brisk walk is about 5 km/h.
Mile per hour mph
1 mph = 0.44704 m/s Exact
Road speed in the US, the UK and a short list of others. Exactly 0.44704 m/s, because both the mile and the hour are exactly defined. To convert in your head, add 60 percent: 60 mph is 97 km/h, close enough to know whether you are speeding.
Foot per second ft/s
1 ft/s = 0.3048 m/s Exact
Feet per second. Used in US ballistics, sports measurement and some engineering, where distances are short enough that miles per hour would be an awkward frame. A baseball fastball leaves the hand at around 140 ft/s.
Knot kn
1 kn = 0.51444444 m/s Exact
One nautical mile per hour, and a nautical mile is one minute of latitude — exactly 1,852 metres. That is the whole point: a ship making 10 knots for six hours has covered one degree of latitude, readable straight off a chart. Never say knots per hour; the per-hour is inside the word already.