Conversion from 3 Miles per hour to Kilometers per hour

Information about the Mile per hour (mph)

The mile per hour is a unit of speed equal to one mile travelled in one hour. Its symbol is mph. It is the road speed unit of the United States and the United Kingdom, and of a small number of territories connected to them, and it is used almost nowhere else.

Britain's position is the odd one. The country converted its shops, its schools and its industry to metric units over the second half of the twentieth century, but left road signs alone. Distances appear in miles and yards, speed limits in miles per hour, and a plan to change them has been announced and abandoned more than once, largely because replacing every sign in the country is expensive and the political return is small.

The number sixty is what makes the unit intuitive to those who grew up with it. At sixty miles per hour a vehicle covers a mile a minute, which turns journey planning into simple mental arithmetic. Speed limits cluster around convenient multiples: thirty in towns, seventy on British motorways, sixty-five or seventy on most American interstates.

Sport and weather keep it alive beyond the road. Baseball pitch speeds, tennis serves and cricket deliveries are quoted in miles per hour in English-language coverage worldwide, and hurricane wind speeds in the Atlantic basin are reported the same way, with the Saffir-Simpson categories defined by thresholds in miles per hour.

Records set in the unit have become cultural landmarks in their own right. The four-minute mile is a pace of fifteen miles per hour, the sound barrier at sea level lies near 767, and the land speed record has stood above 700 since 1997. Everyday driving figures carry the same weight of familiarity: 30 miles per hour in a built-up area, 70 on a motorway, and the observation that a car doing 60 covers 88 feet in a second, which is the number American driving instructors use to explain following distances. Because the unit rarely appears in physics, almost all of its landmarks come from the road, the track and the runway.

One mile per hour equals about 1.609344 kilometres per hour, about 0.44704 metres per second, or about 0.868976 knots.

Information about the Kilometer per hour (km/h)

The kilometre per hour is a unit of speed equal to one kilometre travelled in one hour. Its symbol is km/h, and the forms kph and kmph appear informally although neither is standard. It is the speed unit almost every country in the world uses on its roads.

It is not an SI unit, since the SI prescribes the metre per second, but it is accepted for use alongside the SI and is far better suited to its purpose. Road speeds in kilometres per hour land in a comfortable range of two-digit and low three-digit whole numbers, from a thirty limit in a residential street to a hundred and thirty on a motorway, which is exactly the property a number on a sign needs.

The United States and the United Kingdom are the significant exceptions, both signing roads in miles per hour. The United Kingdom is the more unusual case, since it is otherwise largely metric: distances on road signs are in miles and yards while almost everything else sold in the country is metric, an inconsistency that has survived several attempts at reform.

Vehicle speedometers in most markets show kilometres per hour prominently with miles per hour in a smaller inner scale, a design that dates from cross-border driving in Europe and North America. Rail speeds, cycling averages and weather reports of wind speed in many countries all use the unit, and it is the default in almost every navigation application outside the two exceptions.

Road safety research has given the unit a set of figures that repeat in every campaign. Stopping distance grows with the square of speed, so a car travelling at 50 kilometres per hour needs roughly twice the braking distance of one at 35 and four times the energy has to be dissipated. The probability that a pedestrian struck by a car survives falls steeply between 30 and 50 kilometres per hour, which is why urban limits across Europe have been moving to 30. The same arithmetic explains why a small reduction in a motorway limit produces a large reduction in fuel used, since air resistance also rises with the square of speed.

One kilometre per hour equals about 0.277778 metres per second, about 0.621371 miles per hour, or about 0.539957 knots.