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What is mpg? How is it calculated?

When you’re buying or shopping around for a car, you’ll likely come across the acronym ‘mpg’. But what exactly does it stand for? And how much should you base your car buying decision on it?

What does mpg mean?

Mpg stands for miles per gallon, and is used to show how far your car is able to travel for every gallon (or 4.55 litres) of fuel it uses. As an example, if you own a car that can return 50mpg and its fuel tank only has one gallon of petrol or diesel in it, you should be able to drive 50 miles before the car runs out of fuel.

I’ve also seen l/100km used. What does that mean?

While the UK uses miles per gallon to denote fuel economy, many other countries show how efficient a car is by saying how many litres of fuel it uses after it’s been driven for 100km (roughly 62 miles).

To convert l/100km to mpg, it’s helpful to know that 282.5mpg is the same as 1l/100km. As a result, dividing 282.5 by the car’s fuel consumption in l/100km will give you its figure in mpg. Dividing 282.5 by a car’s mpg figure will also give you its l/100km rating.

For example, the new Volkswagen Golf with the 130hp 1.5-litre petrol engine can return up to 5.6l/100km. Divide 282.5 by 5.6, and that tells us this spec of VW Golf’s fuel economy in miles per gallon is up to 50.4mpg.

If you’d prefer not to break out the calculator, there are plenty of websites that can 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 figure will be.

How is the fuel economy of my car calculated?

The economy figures you find mentioned in car brochures, on carmakers’ websites and in car reviews will have been calculated using the Worldwide Harmonised Light Vehicle Test Procedure – or WLTP for short.

It sounds complicated, but it’s essentially a series of standardised tests that see how efficient a new car really is. Because car models must be WLTP certified before they go on sale in the UK, the test also helps car buyers see how fuel economy figures compare between different cars.

Why are different economy figures quoted for the same car?

Many different phases make up the WLTP tests, to see how efficient a car is at different speeds and in different driving conditions. These phases are:

  • Low  – simulates driving in cities,
  • Medium – simulates driving in towns
  • High speed – simulates driving on rural roads
  • Extra high – simulates driving on motorways
  • Combined – a mixture of the four above phases

For simplicity’s sake, carmakers tend to emphasise the car’s combined WLTP fuel economy figures, as these provide a general overview of a car’s fuel consumption over a variety of speeds and road types.

However, a range of fuel economy figures can often be provided for each phase – as an example, a BMW 3 Series 320i saloon can return between 40.9mpg and 44.1mpg on the WLTP’s combined cycle, depending on its spec. This is because the WLTP tests also factor in the effects that optional equipment and higher-spec trim levels can have on a car’s fuel consumption.

For instance, fuel economy often gets worse if you fit your car with larger optional wheels, as they tend to be heavier, generate more aerodynamic drag and have more rolling resistance (the friction generated as the tyre rolls on the road) than the wheels that came as standard.

Are mpg figures realistic?

While the WLTP tests have been designed to be reflective of real-world driving, they are still carried out exclusively in a lab. As a result, you may still struggle to match your car’s quoted mpg figures while you’re behind the wheel.

Therefore, it’s best to use the mpg figures for comparison purposes, and not as a surefire guarantee of the fuel economy your car will be capable of in day-to-day driving.

This can especially be the case if you drive a plug-in hybrid (PHEV) car. Because PHEVs can be driven for short distances on electric power alone (meaning their engines aren’t running for the entire duration of the test), their official economy figures aren’t always the easiest to replicate – for example, the Mitsubishi Outlander PHEV is apparently able to return a very impressive 139.7mpg on the WLTP’s combined cycle.

While that doesn’t mean you won’t ever be able to match your PHEV’s official fuel economy figures, it is very much dependent on factors such as how long your journey is, how much charge is in the batteries when you set off and how frequently you’re able to stop off at a charging point.

If you are interested in buying a fuel efficient car, take a look at the most economical cars by tapping the link below.