How It Works

Why the UK and US Still Use Miles and Gallons (and How to Convert Them)

October 5, 2026·5 min read

Drive in India and everything is in kilometres, km/h and litres. Cross the Atlantic and the same road trip is in miles, mph and gallons. This isn't an accident of taste: it comes down to when each country standardised its measurements, and how expensive it turned out to be to change roads, signs and speedometers once they were built.

How India switched to metric

India passed the Standards of Weights and Measures Act in December 1956, with metric weights and measures phased in from 1 October 1958. Metric weights became mandatory by October 1960 and metric measures by April 1962, after which other systems were banned for trade. Decimal coinage (100 paise to the rupee) had already arrived in 1957. Because India converted everything in one planned programme, road signs, speedometers and fuel pumps all use km, km/h and litres.

Why the UK kept miles and mph

The UK's imperial system dates from the Weights and Measures Act of 1824, and the statute mile was fixed in England as far back as 1593. Britain began a metrication programme in 1965, and in 1969 the government announced that speed limits would switch to km/h in 1973, but the plan was abandoned. Today UK law still requires speed limits on road signs to be in miles per hour and distances in miles and yards. Cost was a big factor: a 2006 estimate put converting every road sign at hundreds of millions of pounds.

Gallons: why the US and UK versions differ

A US gallon is 3.785 litres, based on the old 231-cubic-inch wine gallon. The UK imperial gallon, set in 1824, is 4.546 litres, about 20% bigger. That's why a car that gets '40 mpg' in a UK review is not the same as 40 mpg in a US review. When you convert a US fuel-economy figure, use US gallons; for UK figures use imperial gallons.

Quick conversions you will actually use

Speed: 1 mph = 1.609344 km/h, so 60 mph is about 97 km/h, and 100 km/h is about 62 mph. Try our KM/H to MPH converter or MPH to KM/H converter. Fuel and liquids: 1 US gallon = 3.785 litres, so a 15-gallon US tank holds about 57 litres. Use Gallons to Liters or Liters to Gallons. Distance: our KM to Miles converter handles the rest.

Frequently Asked Questions

Does the UK use km/h or mph?

The UK uses miles per hour on all road speed-limit signs, and distances on road signs are in miles and yards.

When did India adopt the metric system?

The Standards of Weights and Measures Act was passed in 1956, metric weights and measures were phased in from 1958, and other systems were banned for trade by April 1962.