Fuel station pumps

What is MPG?

Look up how fuel efficient the car you’re thinking of buying is and you’ll see a number given in units of MPG. But what exactly is MPG? Here’s everything you need to know.

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By Cazoo editorial team

What does MPG mean?

MPG is the unit of measure for a car’s fuel economy (also known as ‘fuel consumption’). It stands for Miles Per Gallon. MPG numbers tell you how many miles a car can go on a gallon of fuel.

A car listed as getting 45.6mpg can go 45.6 miles on a gallon of fuel. A car that can do 99.9mpg can go 99.9 miles on a gallon of fuel. It really is that simple.

At Cazoo, we use the ‘official’ average MPG figures published by a car’s manufacturer. Other information sources may use different figures having carried out their own tests.

How is MPG measured?

The procedures for measuring a car’s fuel economy have changed many times over the years. The current one is called WLTP - Worldwide harmonised Light-duty vehicle Testing Procedure. All cars sold in the UK after 1 September 2019 had their fuel economy tested this way. (The previous test procedure was different – we’ll come on to that in a moment.)
WLTP is conducted in a laboratory, but it’s designed to reflect real-world driving. The cars are ‘driven’ on a rolling road - essentially a treadmill for cars. Every car is driven in exactly the same way through a series of acceleration runs, braking events and cruising at different speeds. Sounds simple enough but it’s actually mind-bogglingly complex.

The tests are designed to mimic driving on every type of road, including city streets and motorways. The amount of fuel used is measured and a fairly simple calculation reveals the car’s MPG.

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What’s the difference between NEDC and WLTP?

The previous fuel economy test used across Europe was called the New European Driving Cycle (NEDC). While it was a level playing field since all cars faced the same test, most car owners found that their cars got nowhere near the ‘official’ MPG.

The WLTP numbers are lower (and more realistic). That’s why older examples of certain cars appear to be more economical than more recent examples. The car didn’t change, but the test did.

It’s a potentially confusing situation and it can be difficult to find out whether your car’s MPG figures were produced by NEDC or WLTP. If your car was launched after 2017, it was subject to WLTP. All cars sold after 1 September 2019 were subject to WLTP.

Why are there several different MPG figures for every car?

Car manufacturers release several different MPG numbers for their cars. These numbers are typically called Urban MPG, Extra-urban MPG and Combined MPG and refer to different driving situations.

Urban MPG tells you how much fuel a car will use on a journey driving around town, while extra-urban MPG tells you much fuel a car will use in a journey that includes a bit of town driving and higher-speed A-roads.

Combined MPG is the average. It tells you how much fuel a car will use in a journey that includes every type of road - town, country, motorway. At Cazoo, we give Combined MPG numbers because it’s the one that most closely relates to the kind of driving most people do.

How accurate are official MPG figures?

All official MPG numbers should only be taken as a guide. The fuel economy you’ll get from your car depends on how you drive. As such, you may never get anywhere close to the official MPG numbers, or you could exceed them. In general, the WLTP combined figure should be reasonably close to what you’ll get if your driving habits and style are average.

There are caveats, though. Official MPG numbers for plug-in hybrid cars are often very optimistic. You may see official MPG numbers in the hundreds for such cars but you’re unlikely to get near that in the real world. The discrepancy is because real-world fuel economy is dependent entirely on whether you keep the battery fully charged and how you drive.

How do I calculate my car’s MPG?

Every car has a trip computer that displays the current and long-term MPG. You can reset the trip computer if you want to record a new set of numbers.

The trip computer is a good guide, but it’s not always 100% accurate. If you want to find out exactly what MPG your car is giving, you’ll need to calculate it yourself. Fortunately, it’s not difficult to do.

Fill your car’s fuel tank until the pump clicks off. Take note of the mileage showing on the odometer and/or reset the mileage to zero on the trip computer.

Next time you fill your car’s fuel tank (again, until the pump clicks off), note the amount of fuel that’s gone in. This will be in litres, so divide it by 4.546 to get the number of gallons. Note the mileage on the odometer, or the mileage readout on the trip computer. Divide those miles by the gallons and you’ll have your car’s MPG.

Here’s an example:

52.8 litres ÷ 4.546 = 11.615 gallons

368 miles ÷ 11.615 gallons = 31.683mpg

What does L/100KM mean?

L/100KM is another unit of measurement for a car’s fuel economy. It stands for Litres per 100 Kilometres. It’s used across Europe and in other countries on the metric system. The KM/L unit - Kilometres per Litre - is occasionally used, as well. You can work out MPG from L/100KM simply by dividing 282.5 by the L/100KM figure.

Can I improve my car’s MPG?

A good place to start is by making sure your car is as aerodynamic as possible. Open windows and roof racks, for instance, block the air flowing around the car. The engine has to work a bit harder to push the car along, which worsens fuel economy.

Keeping the tyres pumped up to the correct pressure is vital, too. A tyre with low pressure squashes out, creating a larger ‘contact patch’ with the road. That creates more friction than normal which the engine has to work harder to overcome, worsening fuel economy.

It’s worth noting that the bigger a car’s wheels, the worse its fuel economy will be. A high-spec car with 20-inch wheels may look great, but its fuel economy is often several MPG worse than a lower-spec model with 17-inch wheels because the engine has to work harder to turn the larger wheels.

Your car’s electrical equipment uses power generated by the engine. The more of that equipment you have turned on, the harder the engine has to work, so the worse your fuel economy will be. The air con, in particular, can have a big effect. Keeping unnecessary equipment turned off will improve fuel economy.

But by far the best thing you can do to make sure your car does as many MPG as possible is to have it serviced regularly. If your car’s engine isn’t fit and healthy, it simply won’t be able to give you its best MPG.

Can how I drive affect my car’s MPG?

How you drive can have a big impact on your car’s fuel economy, especially if your car has a manual gearbox.

Revving the engine hard and changing gear at high revs worsens fuel economy. The faster the engine revs, the more fuel it uses.

Likewise, using too few revs and changing gear too early can worsen fuel economy. That’s because the engine has to work harder to accelerate the car. If you’re a cyclist, you may have experienced how difficult it is to get going if your bike is in a high gear. The principle applies to cars, as well.

Every engine has a ‘sweet spot’ where it gives the best balance of performance and fuel economy. That spot is different in every engine, but you should be able to find it fairly easily. Cars with automatic gearboxes are designed to work within their sweet spot at all times.

Most modern cars have an ‘eco’ driving mode you can select at any time. It changes how the engine works to optimise fuel economy.

What cars give the best MPG?

In general, the smaller the car, the better its fuel economy will be. But that doesn’t mean larger cars can’t be fuel efficient.

A lot of larger cars, especially diesels and hybrids, return excellent fuel economy, as much 60MPG or more. If we take 45MPG as a reasonable measure of good fuel economy, you’ll be able to find any type of car that gives you that and still suits your other needs.

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