Guide

Speeds in traffic and in engineering

Traffic uses km/h, physics m/s, shipping and aviation knots, Anglo-American countries mph. All four describe the same quantity but differ by factors that are not always at hand mentally.

The factor 3.6 between km/h and m/s

An hour has 3600 seconds and a kilometre 1000 metres. Therefore 1 m/s is exactly 3.6 km/h. The conversion is pure mental arithmetic: divide by 3.6 from km/h to m/s, multiply by 3.6 the other way. 100 km/h is about 27.78 m/s.

Miles per hour and knots

A mile measures 1609.344 metres, a nautical mile 1852 metres. So 1 mph = 1.609344 km/h and 1 knot = 1.852 km/h. The nautical mile corresponds to one arc minute along a meridian, which is why it fits together with coordinates in navigation. 100 km/h is about 62.14 mph or 53.996 knots.

Reaction distance and braking distance

The stopping distance is the sum of reaction distance and braking distance. The reaction distance grows linearly with speed, the braking distance with its square: at twice the speed the braking distance is four times as long. The rule of thumb for braking distance in metres on a dry road is speed in km/h divided by ten, then squared. Unit arithmetic does not replace driver training.

Average and instantaneous speed

Average speed is the total distance divided by the total time, including all stops. Instantaneous speed is the value at a particular moment. Covering 200 kilometres in 2.5 hours gives an average speed of 80 km/h, even if the speedometer showed 130 km/h at times.

Matching conversions

Matching tables

Unit family: Speed

Sources