New fuel efficiency tests needed to rebuild trust

Diesel emissions are not the only example of misleading claims from manufacturers. For years our testing has proved that fuel economy figures do not match official MPG claims – and VW is not the only culprit.

Official test figures are only as good as the tests themselves. One of the biggest sells for a new car is how efficient it is, measured by how many miles per gallon (MPG) you get.

Following the news that up to 11 million diesel Volkswagen (VW) cars around the globe have discrepancies between claimed and real ‘tailpipe’ emission values, we’ve looked at the MPG values of diesel cars from both the VW Group and its competitors.

Diesel cars are big business in the UK, 48% of the 1.6 million cars registered as new in 2015 have a diesel engine.

The average difference between the claimed and measured fuel economy figure is 12%, meaning you could end up spending hundreds more on diesel every year. The VW Golf is the second-worst offender of this selection, behind the Peugeot 308 which is 17% away from its MPG claim. Only the BMW 1-Series comes close to its official figure, but still can’t quite live up to its claim.   



Richard Lloyd, Which? executive director said:

“Our research has consistently showed that the official test used by carmakers is seriously in need of updating as it contains a number of loopholes that lead to unrealistic performance claims. The car industry needs to focus on how to rebuild trust with consumers and we want to see the new fuel efficiency test introduced as soon as possible.”

Why are claimed and real MPG figures so different?

The problem is the official test that manufacturers have to use under EU law is unrealistic and there are numerous loopholes that can be exploited.

  1. The test cycle only includes urban (in town) and extra urban (out of town) driving, but while it reaches a top speed of 75mph for 10 seconds, it doesn’t include any sustained motorway driving. This is the type of driving where cars usually consume the most fuel.
  2. Many modern cars have adaptable driving modes to make them more economical or sportier. Our testing finds that ‘eco’ modes makes cars feel unresponsive, so while manufacturers may carry out the official test cycle using them, we think most drivers won’t.
  3. The test is conducted with additional energy-consuming features turned off, including air conditioning, lights and heated windows, thereby making the car more efficient.
  4. Roof rails, extra lights and even the door mirror on the passenger side can be removed which makes the car lighter and therefore more fuel efficient.
  5. There is no restriction for the air pressure in the tyres, meaning manufacturers can use higher than recommended pressures to reduce rolling resistance, reducing engine load and fuel use.
  6. There is no official body in place to police the testing procedure and monitor results from lab to lab. Also, the labs are commercial enterprises that rely on the carmakers for business.
  7. All manufacturers follow the same test procedure, but can select any accredited lab to use for the test. It’s very hard to get truly repeatable and comparable results when using multiple labs.
  8. There is a tolerance for the testing to be carried out at 1.2mph below the required speed, meaning less fuel is used, although the speeds used are already quite pedestrian.
  9. If all that wasn’t enough, the rules allow carmakers to arbitrarily knock 4% off the results at the end of the cycle.

Which? tests are more realistic. On top of the official cycles, we include motorway driving, we use the default setup rather than switching to an eco-mode, the air conditioning is switched on and the headlights are left dipped. We also don’t over inflate tyres or remove any weight from the car prior to testing. Find out more about how we test MPG.

VW diesel scandal: consumer Q&A

What cars are affected by this?

Cars with Volkswagen’s Type EA 189 diesel engine, used in models such as the Golf, Passat and Audi A3, are affected by the issue.

The company had already been ordered to recall 500,000 cars in the US but VW has since admitted that the issue affects at least 11 million cars worldwide.

I own one of these cars! What should I do?

Volkswagen has set aside €6.5bn to deal with the cost of the scandal – you should not incur any charges.

In the event of a UK recall, the manufacturer should initiate contact with you. Arrangements will then be made concerning a repair, likely to be undertaken at a dealer convenient for you.

You should be given a courtesy car while any repair work is being carried out.

It’s important to remember that ignoring safety-related recalls can affect the value of your car – ensure that you follow the recall procedures promptly when they are announced by VW.

How did this alleged cheat work?

The illegal software, known as a ‘defeat device’, is designed to artificially reduce noxious emissions in laboratory tests – tricking regulators into under-recording data and incorrectly certifying the vehicles for road use.

This software has allowed VW to hide the fact that its diesel cars are producing pollution up to 40 times the legal limit.

The software works by controlling a device that injects urea fluid into the emission control system, which reduces the amount of nitrogen oxide released.  It can detect when the car is being run under lab conditions, as devices such as anti-collision systems are turned off in this environment. Once the car returns to normal road use, the software switches itself off – otherwise it would quickly run out of the fluid.

How was the issue discovered?

The issue was discovered by International Council on Clean Transportation (ICCT), a small clean-air NGO that noticed discrepancies between their laboratory and real-world tests, which measured tailpipe emissions, of diesel Volkswagen cars in Europe.

In order to validate their findings, ICCT borrowed equipment from a U.S. university to test the cars’ actual emissions in real-world driving in the U.S, where car emission limits are much lower than in Europe. To their surprise the pattern was repeated – while the cars passed lab tests performed by the California Air Resources Board, they failed the real-world tests.

Do other automakers do this?

Nobody knows for sure. ICCT also tested a diesel BMW X5, which passed the tests, but now a number of regulators and environment agencies from all over the world are starting to investigate Volkswagen cars as well as diesel cars made by other manufacturers.

Notes to editors:


  1. Looking for a new car? Don’t be fooled by official figures. Instead check out our car reviews to find out just how much a car really will cost you in fuel.
  2. MPG fantasy & reality: similar-sized diesel cars table. All testing carried out from 2013 onwards.
  1. Diesel cars are big business in the UK. According to the Society of Motor Manufacturers & Traders (SMMT), of the 1.6 million cars registered as new in 2015, 48% of them have a diesel engine.
  2. In the table, we’ve focused on similar-sized cars to the VW Golf, that also have diesel engines, between 1.6 and 2.2 litres in capacity. Models include other similar cars from the VW group, the Seat Leon and Skoda Octavia, plus competing cars from Honda, Mazda, BMW and Peugeot.

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