What is MPG and how is it calculated?

Andrew Brady

Written by

Andrew Brady

What is MPG?
  • We explain all about fuel economy 
  • How are official MPG figures calculated?
  • How to get better MPG

It’s a disappointing reality that no one has yet invented a car that doesn’t consume fuel. If they had, we wouldn’t need to visit fuel pumps to ensure our cars get from A to B, or know about MPG.

Sadly, they haven’t - and whether your car is powered by petrol, diesel or a hybrid, we all need to fork out for fuel.

How frequently you need to top up on fuel and how much it costs depends on your car’s fuel economy or MPG.

What is MPG?

In the UK we measure a car’s fuel economy in miles per gallon (MPG). If your car does 30MPG, it will cover 30 miles on one gallon of fuel. The higher the MPG, the more economical the car and the less you’ll spend on fuel. Check out our guide to the best MPG cars to find the most economical cars you can buy today.

The quantity of fuel a car burns has an impact on how polluting it is. Every car manufacturer must publish the MPG for each of its vehicles. So, car makers are always developing their motors to use less fuel. The higher the MPG, the lower a car’s environmental impact.

How is MPG calculated?

There’s a common way MPG is calculated to give buyers a reliable way of comparing the economy of different cars.

Since the 1980s, the official way MPG was measured was called the New European Driving Cycle (NEDC). From 2017, this has gradually been phased out and replaced with the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

Like NEDC, the WLTP is laboratory-based. But the tests are designed to reflect real-life driving more accurately. This is because car owners have been complaining for years that the official manufacturer MPG figures are too high to be matched when driving on the road.

Nonetheless it’s very difficult to replicate everyday conditions such as wind direction and gradient. And even with WLTP it’s still very difficult for ordinary drivers to achieve official MPG figures.

How to see your car’s MPG

Many modern cars have trip computers that record and calculate miles per gallon. Most give read outs for instant as well as long-term MPG, which you can often see on your dashboard. It’s a quick and convenient way to see how much fuel you’re using. It can also be fun adjusting your driving style to try and increase MPG as much as possible.

How to work out MPG

Car computers are a great way to view your motor’s MPG, but they’re not always accurate. The most precise way of doing it is to keep your own record.

You do this by first filling the fuel tank to the brim. At the same time, record your car’s mileage and zero the trip computer. Every time you fill up after that, get the tank as full as possible. Note down the exact number of litres of fuel you’ve put in, as well as the mileage readouts. You must do this every time you add fuel or it won’t work.

After a period of time – the longer the better – you can calculate your car’s MPG. You do this by converting the total number of litres of fuel you’ve bought into gallons (multiply by 0.219). Subtract the starting mileage from the final figure. Then you divide the number of miles you’ve covered by the gallons of fuel you’ve put in. This gives you miles per gallon.

Is my car fuel efficient?

The newer the car, the more fuel efficient it’s likely to be. You can see how it compares to rivals by comparing their official MPG figures. The usual MPG figure that’s taken is the Combined figure, which is an average of the Urban and Extra-Urban figures.

Extra-Urban fuel consumption

The car industry’s test for this is designed to replicate driving on country roads and the motorway. The car is driven at speeds of between 43mph and 75mph for specific periods of time. The amount of fuel used is then calculated.

Urban fuel consumption

This is supposed to mimic city or town driving. The car drives at between nine and 20mph for a strictly defined time before the amount of fuel used is worked out.

How to get better MPG

The amount of fuel a car uses depends on two things: the car, and how it’s driven. The type of fuel your car uses also impacts the fuel economy - that’s why its important you choose the right fueled car for your needs.

Diesel is great for covering distances. If you have a long commute or regularly do longer trips, it’s probably the fuel for you.

Petrol is better for shorter, stop-start journeys. And if you spend much of your time in an urban environment, a petrol-electric hybrid may well be the best compromise.

But whatever the car, one that’s very economical on paper needs to be driven correctly to get the best MPG. To maximise any car’s potential MPG you must avoid accelerating hard. It’s best to anticipate what other road users are doing well in advance to avoid late braking. And you should change gear before the revs get too high.

Look after your car to get the best MPG

There are several things you can do to get the best possible MPG for your car.

The first is to pump your car’s tyres up. According to tyre manufacturers, fuel economy is reduced by about half a per cent for every pound per square inch (PSI) that the tyres are below the recommended pressure.

A car that’s been properly serviced will also deliver the best possible MPG because it will be running at its most efficient.

Finally, weight is the enemy of good fuel consumption. Take that unused gym kit out of the boot and you’ll reap the reward with improved MPG.