Speed converter
Five units of speed at once. The two worth carrying in your head: 100 km/h is about 62 mph, and a knot is about 1.15 mph.
How to convert speed
The knot is a nautical mile per hour, and since a nautical mile is one minute of latitude, a vessel making one knot covers one minute of latitude an hour, which is what made it worth having before satellites. Aviation and shipping still use it for the same reason. For everyday road work, the useful approximation is that km/h to mph is roughly divide by eight and multiply by five: 80 km/h becomes 50 mph, which is accurate to within about one percent and does not need a calculator.
Questions
About 62.1 mph. The quick mental version is divide by eight and multiply by five.
One nautical mile per hour: about 1.15 mph or 1.85 km/h.
Because a nautical mile is one minute of latitude, so speed in knots maps directly onto position on a chart.
Multiply by 3.6, since there are 3,600 seconds in an hour and 1,000 metres in a kilometre.
A ratio to the local speed of sound, which varies with temperature and altitude, so it is not a fixed unit and is not included.