What is MPG and how is it calculated?

What is MPG?

MPG is a measure of fuel efficiency and indicates the number of miles that your car can travel on one gallon of fuel. This metric can also be used to calculate your car’s running costs.

A car with a high MPG will be able to drive further on a smaller amount of fuel than a low-MPG model.

Using less fuel has two big benefits: fewer CO2 emissions and air pollutants - and more financial savings for you. In a nutshell, the higher your car’s MPG, the cheaper it will be to run and the more eco-friendly it will be.

How is MPG measured?

MPG and vehicle emissions were originally measured using the New European Driving Cycle (NEDC), which was introduced in 1980. This used lab-based tests which were intended to replicate real-world driving conditions. However, the tests didn’t produce accurate results and despite the NEDC being updated to improve accuracy, it was eventually replaced.

Since 2017, MPG has instead been measured using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP). Whilst still lab-based, the WLTP is much more rigorous than the NEDC. It tests at higher speeds and temperatures, uses more varied acceleration and braking patterns, lasts for longer, and accounts for more road types. It also factors in the use of air conditioning, heated seats and so on.

Real Driving Emissions (RDE) testing has also been introduced to test emissions out of the lab and on the road, under a variety of driving conditions.

Different types of MPG figures

The different MPG figures listed below correspond to the different parts of the WLTP tests. They help you understand which car is best for your needs - and paint a more accurate picture of fuel efficiency.

The WLTP test phases are:

• Low: Simulates driving in urban areas at speeds of up to 35mph/56.5kmh. This test emulates driving in a city centre at slow speeds, with lots of idling, stopping and starting.
• Medium: Simulates driving in suburban areas at speeds of up to 48mph/76.6kmh. This test emulates driving through towns and residential areas, with some slow driving, stopping and starting, but in generally light traffic.
• High speed: Simulates driving in rural areas at speeds of up to 61mph/97.4kmh. This reflects cruise-style drives through the countryside (for example, on A roads).
• Extra high: Simulates driving on a motorway at speeds of up to 82mph/131.3kmh. This replicates fast motorway driving with little or no stopping.

Each part of the test results in a different figure. These are then combined to provide a final average figure, which takes all the results into account. This is the one you’ll generally see advertised.

You might find that a range of combined WLTP figures are provided for some vehicles, even those that are only available with one choice of engine. This is because fuel economy and therefore, WLTP test results may be affected by extras such as larger wheels.

Real-world MPG vs Official figures

There will always be a gap between official MPG figures and how many miles per gallon your car actually does on the roads.

This is because the tests are lab-based. So, they can never accurately reflect all the variables of real-world driving (e.g. driving style, weather, road conditions and load weight).

Although the MPG figures produced by the WLTP tests are much more accurate than the NEDC equivalents, they do tend to report slightly higher MPG figures (around 5% higher) than what drivers would experience themselves.

You can get a better idea of your real-world MPG by tracking how much fuel you buy and how many miles you travel. Whilst you can do this manually, you may find it easier to track with an app. We’ll explain the process for tracking your real-world MPG in detail below.

Please note: There are websites that collate data from drivers and provide average real-world MPGs, which may be more accurate than the figures from WLTP tests.

How to calculate your car’s MPG (step-by-step)

• Fill your tank to capacity and set the trip counter to zero.
• When you next fill up, make a note of the mileage you’ve covered since you last refuelled - and how much fuel you’ve used.
• Divide the number of miles driven by the amount of fuel used in litres, then convert this figure to MPG by multiplying it by 4.544.

You can also use our MPG calculator to work out your car’s MPG reading. To get started, simply enter the number of litres or gallons of fuel used - and the number of miles you’ve travelled.

MPG calculation example

If you filled your tank with 25 litres of fuel, then drove 140 miles before you next had to refuel, your MPG calculation would look like this:

140 ÷ 25 = 5.6 miles per litre

5.6 x 4.544 = 25.45 MPG

It’s best to repeat these steps over a period of a few months. Try to be as consistent as possible to obtain a realistic MPG.

Some modern cars can record your MPG on their trip computer. It’s worth checking whether your car has this feature to save yourself the hassle of recording manually.

You can convert MPG to litres per 100km with the conversion ratio 282.481. The formula to convert an MPG reading to litres per 100km is as follows:

Litres per 100km = 282.481 ÷ MPG

As an example, using this formula to convert 5 MPG to L/100km would yield a result of 56.4962 L/100km. It shouldn’t be too complex to work out any conversion result using this rule.

However, if you want to keep it as simple as possible, just use our tool to calculate your car’s MPG.

Tips for improving your car’s MPG

If your car isn’t hitting its expected MPG, try adopting the following hypermiling techniques:

First, look at how you’re driving:

• Don’t brake or accelerate too hard.
• Try to avoid too much stopping and starting.
• Keep idling to a minimum.
• Maintain a consistent speed whenever you can.
• Use cruise control when appropriate.

Next, think about what you’re carrying in your car. Extra weight can negatively affect MPG, so remove anything that you don’t plan to use before each journey. Give your boot and back seat a clear-out for good measure.

Overusing your car’s electrical equipment can reduce your MPG. So, only use your air conditioning, heated seats and infotainment system when strictly necessary.

Finally, make sure you’re maintaining your car properly. Keep the tyres properly inflated, service your car at regular intervals - and use premium fuel if you can.

MPG and emissions

In the current economic and environmental landscape, it’s never been more important to reduce your fuel emissions.

By choosing a high MPG car, you’ll be producing fewer air pollutants and CO2 emissions.

The process of producing and distributing fuel is environmentally unfriendly, so by cutting your fuel usage, you’ll reduce your carbon footprint.

What’s more, fewer top-ups at the petrol station mean more cash in your pocket!

Frequently Asked Questions