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How we test mpg and emissions

By Adrian Porter

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How we test mpg and emissions May 2012

Want to know your car’s real mpg? We reveal why official figures can’t be trusted and how Which? mpg tests are more realistic.

Cars with the best mpg offer the greatest savings on fuel. Which? research has discovered that the difference between the most and least fuel-efficient medium-sized hatchback is more than £800 a year – enough money for a holiday.

But the key to saving fuel is getting to the real mpg figures, as our tests show the advertised figures can’t usually be trusted.

Which? car tests discover not only a car’s true mpg, but also how reliable it is, the usable boot and passenger space, and how easy it is to drive. Find a brilliant new or used car – we reveal the best cars.

Claimed mpg figures explained

Car manufacturers are required to publish fuel economy figures (usually expressed as miles per gallon, or ‘mpg’), along with exhaust emissions data (air pollution), each time they launch a new car. These are the figures you tend to see in adverts and brochures.

These figures all come from the current European Commission test protocol, which has been widely criticised as easy to dupe and not being challenging for modern cars.

Fuel economy figures are split into three different categories:

  1. Urban represents towns and city driving.
  2. Extra urban refers to driving outside of towns and cities.
  3. Combined merges urban and extra urban together to give an overall average. This is the figure you’ll most likely see in adverts and promotions.

The same test cycle is also used to derive how much air pollution, such as NOx (oxides of nitrogen) and particulate matter, cars create while driving. 

You can read more about the different emissions and their effects, as well as find out which cars are the dirtiest, in our car emissions explained guide.

Problems with claimed mpg

Don’t be fooled by the name of the official driving cycle currently used to calculate mpg figures. The New European Driving Cycle (NEDC) hasn’t been updated since 1997, and the basis of the test was first introduced in the 1970s.

The rules allow manufacturers to arbitrarily knock 4% off the results at the end of the cycle.

The test’s numerous loopholes and lack of real-world driving scenarios make the figures it generates unachievable when you actually get behind the wheel of a car. 

Here are some of the most notable:

  • The test cycle 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 for which many cars consume the most amount of fuel in our tests.
  • Many modern cars have adaptable driving modes to make them more economical or sportier. Manufacturers can elect to carry out the official test cycle using an eco mode, but we think few drivers actually opt to use them day to day because they tend to make a car feel sluggish and unresponsive.
  • The test is conducted with all ancillary loads turned off, including air conditioning, lights and heated windows, thereby improving efficiency.
  • 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.
  • Roof rails, extra lights and even the door mirror on the passenger side are allowed to be removed. This makes the car lighter and, therefore, more fuel-efficient.
  • There’s no restriction on the air pressure level in the tyres, meaning manufacturers can use higher-than-recommended pressures to reduce rolling resistance. This takes load off the engine and reduces fuel use.
  • There’s no official body in place to police the testing procedure and monitor the consistency of results between labs.
  • 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, comparable results when using multiple labs.
  • If all that wasn’t enough, the rules also allow manufacturers to arbitrarily knock 4% off the results at the end of the cycle.

New official mpg and emission tests from September 2017

With support from our own ‘Come clean on fuel claims’ campaign, official tests will soon drop the NEDC completely and replace it with the Worldwide-harmonised Light vehicles Test Procedure (WLTP) cycle.

This is not only tougher than the NEDC, but it should also close the loopholes above and is one of the cycles that we have used for years in our own testing.

The new tests come in from September 2017, but manufacturers are being given a year to conform with the new procedures. So it won’t be until September 2018 that every new car sold in the UK will have official fuel figures based on WLTP testing.

How Which? finds a car’s real mpg

As with the official cycle, the Which? fuel economy test is carried out on a rolling road in a lab. This simulates wind resistance and allows the test to be repeated in the same way in the exact same environment, as we use the same lab for each car test.
£800+ The difference in annual fuel costs between the most and least-efficient medium-sized cars, as revealed by Which? tests.

We use the Worldwide-harmonised Light vehicles Test Procedure (WLTP) cycle, running both a ‘cold cycle’, where the engine is cold, and a ‘hot cycle’ where the engine is already warm.

We also have a motorway-driving cycle where we accelerate up to, and sustain, motorway speeds. This cycle catches out a lot of cars that are efficient at low speeds but burn a lot of fuel on the motorway.

If you do a lot of miles, remember to check out the motorway mpg figure that we publish in every one of our independent car reviews.

We also set the conditions of the test to be as realistic as possible:

  • We test cars in the default setting they start up in, rather than switching to a more economical driving mode.
  • Our additional test cycles are conducted with the air conditioning on, lights on dipped beam and the radio working. Our trained drivers also stick to the test’s speed restrictions.
  • We don’t tamper with the cars by removing extra weight or inflating the tyres any more than specified in the owner’s manual.

Finally, we don’t adapt our results in any way at the end of our procedure. What you see in our car reviews is what you get.

What about hybrid cars?

Hybrid cars are subject to the same test cycles so that we can compare them directly to their petrol and diesel counterparts.

For non-plug-in hybrid cars 

The test is conducted with a 60-70% state of charge displayed on the on-board computer.

For plug-in hybrid cars 

We start each test cycle with a full battery charge and continue to repeat them until the state of charge drops below 50%, taking a measurement of the electric energy supplied for charging. We then conduct the same cycles with an empty battery once. Our calculation then takes all of these factors into account.

Thinking of buying an electric car? Choose carefully, as the worst are expensive and impractical. Thankfully, we can help: find out which electric cars are comfortable and can go long distances between charges. See the best electric cars.

What about air pollution?

In the same test, we collect air-polluting emissions data: the quantity of exhaust emissions, such as oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM).

Any vehicle that produces a very large amount of air pollution in our tests cannot be a Best Buy car.

Volkswagen was caught out in September 2015 by employing a ‘defeat device’ in its diesel cars to dupe US tests over NOx emissions. This has become known as the VW emissions scandal, or ‘dieselgate’.

In Europe, 8.5 million cars (1.2 million of which are in the UK) have the same device and are currently being recalled.

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How we stop manufacturers from cheating our tests

There is a risk that cars (from any manufacturer, not just VW) could use similar means to detect they are in a lab testing environment and switch to a more economical running mode. To stop any manufacturer confusing our tests, whenever we are suspicious of the NOx emissions we record in the lab, we now run additional test cycles using a Portable Emissions Measuring System (PEMS).

This device is placed in the car, and the emissions are recorded while driving on real roads. We then compare this NOx emissions data with that recorded in the lab and penalise any cars that have an incredibly high real-life reading.

While this is a realistic way of testing cars, we couldn’t replace our lab testing with PEMS testing alone. Being outside the lab, we are not able to control factors, such as temperature or road conditions, which can affect the result. So results from PEMS are not directly comparable to each other, unlike our lab tests.

Fuel economy you can rely on

While some rival publishers only show the manufacturer’s official mpg figures in their car reviews, and some now offer a limited number of real-world figures, we show accurate mpg data for every car we’ve reviewed. That’s currently more than 700 vehicles, and we test around 120 new cars every year.

Whatever your need, our independent tests reveal the full picture: how fuel-efficient, reliable, easy to drive and comfortable cars really are. To help you find the best new or used car at the right price, see our round-up of the best cars.

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