What Does MPG Mean and What Is It?

If you are looking through the specifications of any new car you will see an MPG figure. This refers to Miles Per Gallon, which is a measure of the vehicle’s fuel economy.

This is of course a little confusing as the petrol or diesel you buy is measured and priced in litres these days, which means you will have to crack out the calculator if you want to work out how much a car will cost you to run.

However, the MPG figure is a benchmark which has been used for more than a century to compare the efficiency of cars. The numbers you see quoted in brochures are the ‘official’ consumption figures, which are measured in laboratory conditions to ensure fairness, and are called the Worldwide Harmonised Light Vehicle Test Procedure (WLTP). You may see this term used in literature too.

The results may be separated into different figures for ‘highway’ and ‘city’ driving or ‘low’ and ‘high’, but the most commonly quoted number is the ‘combined’ figure which is designed to represent driving on a variety of roads.

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Why is the MPG figure important?

Other than helping buyers make an informed decision about their purchase by being able to compare cars on a shortlist, it is also used by the government to determine levels of road tax and how much company car drivers will pay too. This is because the MPG figure and the official CO2 emissions are linked – the more fuel a car burns, the higher its emissions. 

What is a good MPG figure for a diesel car?

A small city car such as the Hyundai i10 has an official figure of around 67mpg. If you are looking at plug-in hybrid cars such as a Vauxhall Astra or MG HS then it gets a little more complicated, as the official MPG figure includes some driving with electric assistance. This means it may look unusually high – an Astra’s official combined MPG is 256, for example.

Diesels will usually have better MPG figures than a non-hybrid petrol car, but the gap is narrowing and as petrol is cheaper to buy at the pump, you may need to do some sums to work out what is best for you. The experts at Eden will be happy to help you crunch the numbers.

How can I work out how much my car will cost to run using an MPG figure?

In the most basic terms, a car which is doing 50 MPG will be able to drive for 50 miles on one gallon of fuel. A gallon of fuel is equal to 4.56 litres, so if a litre of petrol is £1.80 then a gallon costs £1.80 x 4.56 which equals £8.21. Divide that by 50 and your car will cost 16p a mile to fuel. That will allow you to estimate the cost of doing a long journey or perhaps compare the cost of commuting to other modes of transport.

What is the equivalent of MPG for electric cars?

Obviously pure electric cars like the MG4 or Hyundai Kona Electric don’t use petrol or diesel at all, but it is still possible to compare the efficiency using official, independently tested figures. The most common measure is miles per kWh (m/kWh) which measures the distance travelled using one kilowatt of energy. This can be worked out easily by dividing the official WLTP range figure by the battery size. Again, if you need any help understanding the figures then just ask an Eden expert.