What is MPG in cars?

November 03, 2022 by

When you’re buying or shopping around for a car, you’ll likely come across references to MPG. But what is MPG? And how much should you base your car buying decision on it?

MPG meaning

MPG stands for miles per gallon and is used to show how far your car can travel for every gallon (or 4.55 litres) of fuel it uses. For example, if you own a car that returns 50mpg and its fuel tank only has one gallon of petrol or diesel in it, you’ll drive 50 miles before the car runs out of fuel.

How to work out MPG

Official fuel economy figures are calculated based on WLTP testing (which stands for Worldwide Harmonised Light Vehicle Test Procedure).

This is conducted in a lab to simulate real-world driving scenarios, with on-road testing also a component for emission assessment.

If you’re interested in finding out the exact MPG your current car is achieving, the vast majority of new cars will give you a readout of its current MPG on the driver’s display or central infotainment system.

You can also reset your car’s trip computer when you fill up with petrol or diesel, then when you next fill up make a note of how many litres fit in the tank, convert this into gallons (1 litre is 0.22 gallons), and find out how many miles your car has travelled on that fuel.

Why does every car have multiple MPG figures?

WLTP tests are conducted in different ‘phases’ to simulate specific types of driving. Each phase gets its own stated figure, with a final ‘combined’ figure providing an average across all scenarios. This is the figure you’ll most likely see advertised.

  • Low – simulates driving in urban areas at speeds up to 56.5km/h (about 35mph)
  • Medium – simulates driving in suburban areas at speeds up to 76.6km/h (about 48mph)
  • High speed – simulates driving in rural areas at speeds up to 97.4km/h (about 61mph)
  • Extra high – simulates driving on motorways at speeds up to 131.3km/h (about 82mph)
  • Combined – a mixture of the above. This is the figure usually most prominent in advertising.

Sounds simple enough, right? Well, there’s more. You might see a range of ‘combined’ WLTP figures provided for each car – even those that only come with one choice of engine. This is because fuel economy (and the WLTP test results) can be affected by specific extras fitted to some cars.

For instance, fuel economy often gets worse if you fit your car with larger optional wheels, as they tend to be heavier, generate more aerodynamic drag and have more rolling resistance (the friction generated as the tyre rolls on the road) than the wheels that came as standard.

Are MPG figures realistic?

WLTP tests are more reflective of the real world than the old NEDC regime they replaced, but they’re still predominantly laboratory based, so the MPG figure you get may not match what the car is technically capable of. Driving styles, traffic, hills and other factors all affect a car’s economy, therefore, it’s best to use the MPG figures for comparison between cars rather than a surefire guarantee of the fuel economy your car will be capable of in day-to-day driving.

This can especially be the case if you drive a plug-in hybrid (PHEV) car. During their WLTP assessments PHEVs tend to be driven in electric mode a fair amount, which can result in 200mpg+ official figures. To get close to this in the real world you will need to plug in and recharge a PHEv’s batteries on a regular basis, and drive around mainly in electric mode.

Why is a car’s MPG important?

A higher MPG means less fuel is consumed as you’re driving. That means it’ll cost you less to run and (generally speaking) your car will produce fewer emissions as it burns fuel more efficiently. We have a handy fuel price checker that helps you find the cheapest petrol and diesel prices in your area, so you can save money at the pumps every time you need to fill up.

If you’re looking to improve your own MPG, you can influence it with a few changes to your driving habits. Smoother acceleration and braking will help you get that figure up, for example.

What is my car’s MPG?

If you want to see your car’s own officially-claimed MPG, a brochure of your model should provide the answer.

As mentioned earlier, most cars now will give you a readout of the MPG you’re really achieving, too.

If you want to work it out yourself, make a note of the miles you’ve covered on a full tank of fuel and divide that by the number of gallons it takes to refill your tank.

Can I improve my car’s MPG?

Yes: there are a number of things you can do to improve a car’s economy. Not running the air-conditioning is one option (though be sure to turn it on periodically to keep the system in good working order).

Perhaps the largest impact will come from how heavy or light you are with your right foot. Acceleration uses the most fuel when driving, so avoid mashing the pedal to the floor, and try to anticipate what traffic is doing to avoid slowing down, only to have to speed back up again, unnecessarily.

If your car has a manual gearbox keep it in as high a gear as possible, changing up early and avoiding revving the engine more than you need to.

Take any roof box or bars off your car when they are not being used, as these create aerodynamic drag, which requires the engine to burn more fuel to overcome. Also take out any items from the boot you don’t need, as the more mass your car carries, the more fuel it will burn. Be sure to keep your tyres at the correct pressure, too, as under-inflated tyres worsen fuel economy.

When cruising on a motorway or dual carriageway, consider driving at a steady 60 or 65mph rather than 70mph, as higher speeds use more fuel. Our guide to improving your fuel economy has more suggestions.

Does MPG affect my emissions?

Yes, in general, because emissions are waste gases from burnt fuel, so the more fuel the car burns, the more emissions it will produce. That’s not to say all engines are created equal, though: one 2.0-litre four-cylinder petrol engine producing 180hp might emit more CO2 that another engine of identical specifications, simply because the manufacturer has designed and built a better, cleaner engine.

FAQs: Miles per gallon (MPG)

What is a good MPG?

A good MPG is subjective depending on the car and fuel type, but it’s always the case that the higher the number, the better.

Is a higher MPG better?

A higher MPG means your car is consuming less fuel as you’re driving. Therefore, the bigger the number, the better.

Can my driving style affect my car’s MPG?

Your driving style can have a huge impact on your MPG. Smoother driving tends to lead to a higher figure, while jerky reactions and aggressive acceleration will see it drop.

I’ve also seen l/100km used. What does that mean?

Litres per 100km (l/100km) is the European standard for fuel consumption. It’s almost a reverse of MPG — the lower the number is better in this case.

For example, 1mpg is approximately the equivalent of 282.48 l/100km, with 1 l/100km the equivalent to 282mpg. A car achieving 40mpg would return about 7.06 l/100km.

What re the best cars for MPG?

Our guide to this topic offers a full rundown on this topic, but in short think small, light, modern cars, possibly with some form of electrification, such as that offered by hybrid cars.