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MPG Explained: What Is It And Why Does It Matter?

We all know that a vehicle with good fuel economy is better for all of us in the long run, both financially and environmentally, but what, precisely, is mpg? How is it measured and what are the factors which affect it in everyday driving situations? Well, here’s our handy guide with everything you need to know about mpg – and how to choose the best fuel type for you.

What Is MPG?

MPG stands for ‘miles per gallon’. It is an accepted metric to measure how much fuel a car will use to travel a certain amount of distance: the higher the mpg figure, the better. Bear in mind that, here in the UK, that’s miles-per-imperial-gallon, which is slightly different to the US mpg figure, because a US gallon is – believe it or not – slightly smaller than a UK gallon, so as a result quoted US mpg figures for a given car are lower than they would be rendered in UK mpg.

There are alternative measures for fuel economy – the common one on the continent is litres/100km, or 62.1 miles, which in turn informs the most popular electric vehicle (EV) ‘consumption’ measure of kilowatt-hours (kWh)/100km (62.1 miles) – but over here we’re much more focused on mpg alone.

How Is MPG Measured?

All new cars, be they pure internal combustion-powered, some type of hybrid or pure EVs, have to go through an official process called the Worldwide Harmonised Light Vehicle Test Procedure, which is abbreviated to WLTP. This is a laboratory test that every model of car must go through before it goes on sale, to determine how fuel economical it is (or how much electric it uses, in the case of EVs) and also how many exhaust-tailpipe emissions it will put out, in the form of CO2, NOx and more. EVs have zero tailpipe emissions, by the way.

The WLTP was introduced in 2017 to replace the former process known as the New European Driving Cycle, or NEDC, which was how economy and CO2 figures were recorded prior to that point. However, in the wake of the Volkswagen Dieselgate scandal, it came to light that the NEDC gave back wholly unrealistic mpg figures because it was based on a single-cycle lab routine that didn’t really replicate how cars were driven on the roads.

WLTP, however, is more representative, if still not 100% accurate for everyone – the quoted mpg figures are a guide, not an absolute, to a car’s fuel performance. Anyway, the WLTP routine involves 4 different test cycles: Low (up to 35mph), Medium (up to 47mph), High (up to 60mph) and Extra-High (up to 81mph), designed to respectively simulate city, suburban, main road and motorway driving. From these four figures, an overall average is taken and recorded as the combined mpg – it is the combined mpg which most people look at first and which car reviewers also quote.

Which Factors Affect MPG?

Obviously, the construction of the car in question greatly affects mpg. Physically more compact, lighter cars with small, low-powered engines will give you far more fuel economy than big, high-powered machines and high-riding SUVs. Similarly, size for size and power for power, a diesel engine is more economical than a petrol, although the gap between the two is much smaller than it once was.

Moving on, mild hybrids, full hybrids and plug-in hybrids are even more economical and emit less from their tailpipes than pure petrol or diesel models – with plug-in hybrids recording super-high mpg figures and low CO2 levels that help with various tax breaks and VED.

Beyond whether your vehicle is a 1.0-litre, 3-cylinder supermini or a 12-cylinder, 2-tonne SUV, however, there are certain things you can do to get the best fuel economy from a car. Naturally, faster, more aggressive driving involving lots of rapid acceleration and heavy braking will consume more fuel than driving gently. Urban driving, with stop-start traffic, gives back far worse fuel economy than cruising along a motorway at a steady speed. Under- or over-inflated tyres will result in greater fuel consumption, as will regularly carrying needless extra weight (junk in the boot etc) in the car or driving with anything that increases aerodynamic drag (an empty roof rack on the car, all the windows down etc).

Also, while it sometimes cannot be helped due to outside conditions, anything which places a drain on the car’s conventional electrical system (and its alternator) will reduce a vehicle’s fuel economy. This includes the headlights, the car’s heating and/or air-conditioning system, heated seats and/or steering wheel, the in-car sound system and even the windscreen wipers. Therefore, don’t needlessly have the passenger-side heated seat blazing away if it’s the middle of summer and there’s no one even sitting in the seat in the first place, because you’re using extra fuel for no good reason.

What Is A Good MPG Figure?

For a turbocharged petrol engine in a city car, supermini or family hatchback, you should be expecting in excess of 40mpg and up to 60mpg combined economy. Turbodiesels and mild hybrids/full hybrids tend to be more like 50-70mpg combined, while plug-in hybrids will frequently claim a combined economy of 140-230mpg.

This is because the plug-in hybrids have a full battery when they go through the WLTP cycle, meaning they perform much of the test in electric mode without using their onboard combustion engine. This greatly inflates their combined economy figure and it is worth being fully aware that in order to achieve the manufacturer’s quoted fuel numbers for any plug-in hybrid, you need to do just that – plug it in, a lot, and drive it mainly on electric power only. Otherwise, as most plug-ins are petrol-electric vehicles (there are a few diesel-electrics), then you will likely see 40mpg or less from a plug-in hybrid using its engine all the time. This is because the electrical equipment on board is very heavy, so it’s like asking a standard petrol car to lug around 200-300kg of stuff in its boot at all times. 

Which Fuel Type Should I Choose?

When considering petrol versus diesel, the rough rule of thumb is 10,000 miles per year: any more than this, and diesel is going to suit you much better than an equivalent petrol car; less, and you’re better off with petrol, as it is cheaper to buy as a fuel in the first place.

Mild hybrids really suit everyone as they drive almost identically to petrols and diesels in practice, while full hybrids are good for city driving as that is where they will most regularly switch into electric running – bear in mind full hybrids are not designed to travel for much more than a mile or 2 in zero-emissions mode, and even then it will only be at lower speeds and when coasting.

The contentious format is the plug-in hybrid. As the current crop of plug-ins tend to have fully electric ranges of 30-50 miles, then those who will get the best of them will perhaps be commuters who drive up to 50 miles to work and back every day, regularly charging the car via the mains on a daily basis to maintain electric range and occasionally driving longer distances at weekends on the petrol-electric reserves. If you frequently do 200 miles and more on a motorway, and you don’t have the capacity to plug the car in every day to recharge, then a plug-in hybrid might not be for you if you’re after the best possible mpg.

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