What is MPG and how is it calculated?

Last updated June 20, 2022

Fuel economy is an important factor to consider when you’re choosing a new car. That’s partly because fuel can be one of your biggest ongoing car running costs and the better the fuel economy of your car, the cheaper your fuel costs will be over the long term.

Fuel economy is measured using a metric called miles per gallon (or MPG). Understanding what MPG is and how it’s calculated can be useful when you’re in the market for a new car, making your decision of which one to pick a little easier.

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What does MPG mean?

When you’re in the process of searching for and buying a new car, you’re bound to come across the term ‘MPG’. MPG stands for miles per gallon and it refers to a common measurement of fuel economy - the average number of miles a vehicle can drive on one gallon of fuel. The higher the MPG, the more economically friendly a car is.

Basically, a high MPG means you can drive the same distance with less fuel in the tank. This in turn means you should have to refuel less often than vehicles with a lower MPG - providing you have the same size fuel tank, of course. A high MPG additionally has a knock-on effect of the eco-friendliness of a vehicle, which is important to bear in mind.

Both the practical and environmental impacts of a high MPG make it a desirable feature for a car to have - something manufacturers have noticed, as newer cars tend to have higher MPG.

How is MPG calculated?

MPG is rigorously tested by an official authority to give buyers a way to compare fuel economy between certain cars. This process is called the Worldwide Harmonised Light Vehicle Test Procedure (or WLTP, for short).

This is a lab-based test that replicates driving situations that are likely to occur in real life, using different speeds and conditions to test the car’s fuel efficiency. However, it can be a challenge to accurately replicate things such as wind speed or gradient that can affect fuel economy on the road in the lab. Because of this, WLTP figures should be taken with a pinch of salt.

WLTP figures tend to report slightly higher MPG (around 5%) than what drivers would find for themselves. The WLTP figures should be used for informative and comparative purposes only and not as a guarantee of the fuel economy your car will be capable of.

Why does every car have multiple MPG figures?

WLTP tests are conducted by simulating specific driving situations. These are:

  • Low: Simulates driving in urban areas at speeds of up to 35mph/56.5kmh
  • Medium: Simulates driving in suburban areas at speeds of up to 48mph/76.6kmh
  • High speed: Simulates driving in rural areas at speeds of up to 61mph/97.4kmh
  • Extra high: Simulates driving on a motorway at speeds of up to 82mph/131.3kmh

Each part of the test results in a different figure, with these figures combined to provide a final average figure taking all results into account. This figure 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. The reason for this is that fuel economy and therefore WLTP test results may be affected by the extras fitted to certain cars, such as larger wheels.

How accurate are published MPG figures?

WLTP figures are calculated in a carefully controlled environment, designed to eliminate the impact of variables such as the driving style of individual real-life drivers. This means that the resulting MPG figures are generally very effective indicators of how efficient a vehicle is.

Of course, the journeys taken on in reality are far less predictable – harsher driving conditions, varying road quality, etc. – than those used in WLTP tests. As a result of this, some motorists find that their vehicles are unable to achieve the level of efficiency indicated by MPG figures – but it certainly shouldn’t be too far off.

Put simply, you can never guarantee that the advertised MPG figures will turn out to be a 100% accurate reflection of a car’s efficiency levels. However, the level of attention to detail and the elimination of variables during the testing process does mean that the figures are generally an effective way to compare the levels of fuel consumption and energy efficiency between different vehicles.

How can I work out the MPG of my vehicle?

The most reliable way of working out the MPG of your vehicle is to keep your own record, by following these steps:

  • Fill your tank as full as possible and zero the trip counter
  • When you next fill up, make a note of the amount of mileage you’ve covered since last refuelling and how much fuel has been used
  • Divide the number of miles driven by the amount of fuel used in litres, then convert this figure to miles per gallon by multiplying it by 4.544

If you fill your tank with 25 litres of fuel and drive 140 miles before you next have to fill up, then the sum would be as follows:

  • 140 divided by 25 = 5.6 miles per litre
  • 5.6 x 4.544 = 25.45 MPG

It is best to repeat these steps over a set period of time, such as a few months and try to be as consistent as possible to reflect a realistic MPG. Some modern cars will also record your MPG on the trip computer, so it is worth checking to see if your car does to save yourself having to record it manually.

How to convert MPG to L/100km

To convert MPG to litres per 100km, the conversion ratio is 282.481. This means that the formula to convert a MPG measurement 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, it’s worth using one of the many MPG to L/100km calculators you can find online. Just ensure that the conversion ratio is correct each time.

The importance of car fuel efficiency

Fuel efficiency, indicated by high MPG, means that as a driver you’ll be able to save money on fuel. It’s also more eco-friendly, requiring less fuel that is environmentally unfriendly to produce and distribute in order to travel the same distance. However, fuel efficiency depends on more than MPG alone – it also relies on other factors like how you drive and the type of fuel you use.

How to improve MPG

  • Try to increase your car’s fuel efficiency by ensuring that your tyres are always properly inflated, the car has been regularly serviced and you’re not carrying excess weight

  • You can maximise your car’s potential MPG by changing the way you drive (avoid accelerating or breaking too hard, or switching gear in a comfortable zone before the revs get too high)

  • Fuel efficiency and environmental friendliness are not mutually exclusive when it comes to driving a car, so you can follow guidance on how to lower car emissions which should also help your fuel to last even longer