True fuel economy: how to check the real mpg figure of a car

driver screaming at their credit car while filling up with fuel due to excessive costs caused by not knowing true MPG

Imagine buying a new home. You’ve slogged away at work for years, scrimped and saved, given up holidays, ditched meals out and takeaways, and even your shoes are starting to wear through. Finally, the keys to the house of your dreams are in your hand. But there’s a problem. One of the bedrooms is missing. You paid for a three-bedroom house but bizarrely, now it’s yours, there are only two.

You’d be flabbergasted if this happened to you. But increasingly, it’s how millions of drivers are left feeling, after buying new cars, seduced by the promise of being able to travel hundreds of miles on barely a whiff of fuel, only to find that the car is nowhere near as economical as its manufacturer claimed.

The AA says that the disparity between the mpg a car is claimed to achieve by manufacturers and the actual fuel economy it returns in day-to-day driving is growing rapidly.

In 2011, the AA estimated that the average difference between claimed and true mpg was just eight per cent. Most drivers could probably forgive that. However, by 2014, that average gap had grown to a whopping 40 per cent.

It means that someone buying Britain’s most popular car, the Ford Fiesta 1.0 EcoBoost, would achieve just 39.4mpg instead of the official brochure figure of 65.7mpg. Over a year and 10,000 miles of driving, that’s equivalent to a driver losing £545, based on current average petrol prices of 117.9p a litre. And during the average three-year ownership period, it’s a £1635 loss.

What can drivers do to avoid being left out of pocket?

Why is there a difference between claimed mpg and real-world mpg?

The fuel economy figures published by car makers are obtained from EU testing, known as the New European Driving Cycle (NEDC). Conducted in a laboratory environment, it is meant to represent typical driving habits, but in reality, it bears little relation to how most of us drive.

Power-sapping electrical features drivers love to use are switched off

It was developed in the 1970s and features short runs and precious little motorway driving. And because it features roughly 10 per cent of idling time, it particularly suits cars with stop-start systems or hybrid models that shut off their combustion engine.

What’s more, the power-sapping electrical features drivers love to use – from the stereo to climate control, navigation to windscreen wipers – are switched off, but were they used they’d increase fuel consumption.

Is a new fuel consumption test being introduced?

The NEDC test will be replaced in September 2017 with the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

It will last 30 minutes (10 minutes longer than NEDC), leave the car stationary for less time, reach higher top speeds and require faster acceleration.

Will it be more reflective of everyday driving habits? Nick Molden is a fuel economy and emissions expert, and chief executive of Emissions Analytics. The company has tested on-road emissions of thousands of vehicles, and Molden agrees with the AA, saying real-world economy and emissions are typically 39% above the official levels. “We think the introduction of WLTC will about halve that gap,” said Molden.

How to check the true mpg of a car

red citroen c3 parked on street

Happily, because of the efforts of independent companies, it is possible to find out the real-world mpg and emissions of a car. And interestingly, some cars do a much better job of matching their makers’ claims than others.

Both Emissions Analytics and What Car? test cars on UK roads, on the same route using sophisticated measuring equipment that helps drivers replicate prescribed driving conditions.

For example, the sensible Citroën C3 supermini returned 62.9mpg, when tested by Emissions Analytics. That’s a 15 per cent shortfall. And the humble Suzuki Celerio, a small hatchback, has returned 57.8mpg in the hands of What Car’s testers, which just 12 per cent down on the car maker’s official combined fuel economy figure. Both offer an online search facility, so drivers can gain a clear picture of how economical a car is likely to prove – before they hand over their hard-earned money.

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James Mills

James is a motoring journalist and former magazine editor at BBC Top Gear and Auto Express. He has scooped, reported on and reviewed most new cars of the past 20 years, and currently contributes to the Driving section of The Sunday Times.