What is mpg? How is it calculated?

The acronym mpg stands for Miles Per Gallon – a measure of how far a car can travel if you put just one gallon of petrol or diesel in its tank. This standardised figure helps you compare cars based on their efficiency but doesn’t always reflect their real-world fuel economy.

For more information on running your new car, read our detailed guide to UK Road Tax Bands and our handy explanation of MOTs.

What does mpg mean?

A car’s mpg figure will tell you approximately how far it’ll travel using a single UK gallon (4.55 litres) of fuel. For example, a car capable of 45mpg will drive for around 45 miles on a single gallon before spluttering to a halt. Drive at very high speed or in heavy traffic and you’ll cover fewer miles before needing to fill up.

How do I convert mpg to l/100km?

Imperial units, such as miles and gallons, are rarely used in Europe – instead, litres per 100 kilometres (l/100km) is a far more common measure of a car’s fuel efficiency.

To convert l/100km to mpg, it’s helpful to know that 282.5mpg = 1l/100km. As a result, dividing 282.5 by the car’s fuel consumption in l/100km will give you its figure in mpg. The reverse is also true – divide 282.5 by a car’s mpg figure and you’ll find its l/100km rating.

For example, an Audi A1 fitted with the 1.4-litre TFSI petrol engine achieves a fuel economy figure of 4.8l/100km. To find its mpg figure we simply divide 282.5 by 4.8 giving us 58.9mpg.

If you’d prefer not to break out the calculator, there’s a range of websites that’ll reliably convert these figures for you. Put simply, the more fuel efficient a car is, the higher its mpg figure and the lower its l/100km number will be.

It’s important to remember that imperial (UK) gallons are not the same as US gallons – a UK gallon equals 4.55 litres while a US gallon equals 3.79 litres. As a result, mpg figures for North American cars sometimes appear lower than those sold in the UK and Europe.

How is fuel economy calculated?

In Europe, most fuel consumption figures are measured using what’s called the New European Driving Cycle (NEDC) – a system that aims to provide consumers with a fair comparison of all cars available for sale.

Four figures can be calculated from the test:

  • Urban fuel economy – simulates city driving
  • Extra-urban fuel economy – aims to replicate motorway journeys
  • A combined figure – a balance of urban and extra-urban figures
  • Carbon Dioxide (CO2) emissions – measured in g/km

To ensure the fairest comparison possible, all cars are tested in air temperature between 20 and 30 degrees Celsius, and all engines must be started from cold.

What is the Urban cycle?

The Urban cycle test mimics town or city driving and involves accelerating and decelerating slowly up to speeds of nine, 15 and 20mph respectively. This procedure is repeated for 780 seconds, during which the car will have driven around 2.8 miles in a simulated environment.

What is the Extra-urban cycle?

The Extra-urban cycle is designed to replicate driving on country roads and motorways. The car will gently accelerate to 43mph, maintain that speed for 50 seconds, drop to 30mph for 69 seconds and accelerate back to 43mph.

After a further 50 seconds, the car must speed up to 60mph for 35 seconds before briefly reaching 75mph for 10 seconds and then come to a complete stop.

What is the Combined cycle?

The combined cycle figure is calculated as the fuel consumption during both Urban and Extra-urban cycles run consecutively over a simulated distance of just more than 11km (around 6.8 miles).

Are mpg figures reliable?

Official mpg figures should be used for comparison purposes only and are rarely representative of real-world fuel economy. Not all cars will be capable of reliably returning their advertised mpg figure, regardless of how carefully and considerately you drive.

Additionally, the test’s short official test cycles and gentle acceleration requirements often favour hybrids over conventional petrol or diesel cars. Vehicles such as the BMW i8 and Mitsubishi Outlander PHEV can complete these tests under electric power alone and return hugely impressive – but not necessarily representative – figures as a result.

The most fuel efficient cars

If you want to buy a new car that’ll save you money in fuel, then look at our lists of most fuel efficient family cars, seven-seater SUVs, and even sports cars. Still not sure what to buy? Let our car chooser tool narrow down your search or head over to our new car deals page.