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What are WLTP and RDE? New fuel tests explained

New fuel tests are designed to offer a better reflection of the fuel economy you'll see under real-world conditions. Here's what you need to know

Words By Jim Holder

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Legislators are changing the way the emissions and fuel efficiency of new cars are measured from later this year, with the aim of providing car buyers with far more accurate information about the real-world mpg their cars can achieve.

The tests – which you are likely to hear more about in the ramp up to them launching in the autumn of 2018 – have the catchy names Worldwide Harmonised Light Vehicle Test Procedure (WLTP) and Real Driving Emissions (RDE).

The net results of these tests is that cars will be rated on paper as being, on average, around 22% less efficient than they were – although, of course, they will still achieve the same results in the real world.

In addition, because of the extra work needed to conduct the tests it is likely that car makers will offer less model variants (because most will now have to be ratified separately). Examples include manufacturers removing roof rails or sunroofs as options, because of how they modify the aerodynamic profile of the car.

Some models such as the BMW M3 are also struggling to meet the new emissions requirements in the real world, or require costly modification work to do so, and are subsequently being withdrawn from sale.

What is the Worldwide Harmonised Light Vehicle Test Procedure (WLTP)?

The WLTP test is a much more rigorous and complex version of the tests that have been in use (with developments) for around 30 years. Experts estimate that the new test cycle takes manufacturers twice as long to complete as the outgoing New European Driving Cycle (NEDC).

WLTP testing is still conducted in a laboratory, but the challenges the car goes through are much more realistic to the real world.

For instance, the test is now 30 minutes long rather than 20, more than twice as long in terms of distance (now 14 miles rather than seven miles), features a greatly reduced amount of time at a standstill (13% of the test rather than 23.7%), a higher average speed (29mph rather than 21mph) and a regulated and more realistic test temperature (14 degrees in Europe rather than the 20-30 degree range currently allowed).

What is the Real Driving Emissions (RDE) test?

The WLTP test is also being supplemented by Real Driving Emissions (RDE) testing, which as its name suggests takes place on public roads. These tests last 90-120 minutes and must include a strict, equal measure of town, urban or rural and motorway driving. Elevation changes are also mandated, as are testing temperatures.

However, because the accuracy of the test equipment that must be carried by cars undergoing RDE tests is currently not reliable enough, authorities are using it to only verify certain pollutants such as NOx, rather than calculate fuel economy and CO2 figures from it.

In time, as the equipment improves, the allowable difference between the results from the WLTP lab test and open-road RDE test will be narrowed, putting pressure on car makers to ensure that the figures they declare are realistic for car owners.

Will I be able to achieve the claimed figures of the new emissions tests?

While they provided a repeatable benchmark for comparison, one of the biggest criticisms of the outgoing NEDC tests was that it was nigh-on impossible to match the results in the real world.

The new test procedures should be more achievable. Mike Hawes, the chairman of the Society for Motor Manufacturers and Traders (SMMT), says: β€œThis is an opportunity to reassure consumers that their cars will achieve figures much closer to the official ones.”

However, while the mpg results will be an average of 22% lower than before, that’s no guarantee that car owners will be able to replicate the exact figures. As now, variables such as driving style, car weight and preparation, road conditions and weather conditions will still play a significant role in determining what’s achievable.

What does this mean for me?

Of course the car you own now will be as fuel efficient as ever - the test processes only change its on-paper performance.

However, it remains to be seen if the Government will attempt to tax cars based on the revised, harsher CO2 calculations, potentially pushing them up several VED bands and raising company car costs.

The expectation is that this will happen, but not for a few years, in the expectation that current car owners will be able to replace their vehicles by then, in the expectation of changes to the regulations.

The other knock-on effects of the new rules are that there will - in the short-term at least - be less model varients sold because of the massive increase in work required to test each car, and that some cars will be taken off sale as their real-world performance can't meet the new criteria.

It is likely, too, that cars will be re-engineered in minor ways to better suit the needs of the new tests rather then the outgoing ones. Under the old regulations, for instance, gear ratios would be selected to achieve the best possible mpg figures for the official test route. The changed procedures will likely require revisions.

Why were the new emissions tests introduced?

Lawmakers and car manufacturers have been working towards modifying the NEDC tests for more than a decade.

That passage of time hints at both the dissatisfaction with the old test process and the difficulty in coming up with a new test cycle that both sides felt was fair and achievable.

As such, while it is tempting to link the new test processes’ arrival to the Volkswagen Dieselgate scandal, the two are largely co-incidental. However, since that scandal any resistance from the car manufacturers has largely been swept aside.

What does this mean for Lombardyexperience?’s True MPG testing?

Lombardyexperience? has been carrying out its own True MPG emissions and economy tests for years, and while the new WLTP and RDE tests are an improvement on their predecessor, we still believe True MPG is a better barometer of what’s achievable in the real-world, as well as being a more consistent approach to testing.

More details on our test processes, which have been modelled and carried out with input from test experts at Millbrook, can be read here.

Because they use a mapped, real-world test route in laboratory conditions they are repeatable to an accuracy of less than 2% – the variables of the RDE test cycle mean that such accuracy isn’t possible under that test cycle at present.

However, the new WLTP and RDE test figures will provide comparable analysis of every car on sale, and should act to reassure customers of the veracity of the test results.

So how do I know what type of car to buy?

Well, the new fuel tests should make it easier for you to decide whether a petrol, diesel, hybrid or fully electric car will best meet your needs. And if you need more help, then our new What Fuel? tool can offer advice – all you have to do is answer four easy questions.


And don't forget, once you know what type of car you want, our new car buyer marketplace can help you find a great deal.

Next: The most (and least) fuel efficient SUVs >

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