What are WLTP and RDE? Official fuel tests explained

The worldwide test procedure for diesel cars and vans provides consumers with more realistic information on the pollutants and CO2 produced by their vehicle's engine and their fuel or energy consumption.

There are two measures which replace the old (and now confusingly named) New European Driving Cycle (NEDC) tests. The Worldwide Harmonised Light Vehicle Test Procedure (WLTP) applies to all new cars registered from 1 September 2018, and the Real Driving Emissions (RDE) test is mandatory for all cars on sale from 1 September 2019.

The net results of the 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.

Instead of just testing one version of each model, the WLTP tests must be carried out for every model variant. This is likely to result in less choice for consumers, both of different trim levels and of optional extras.

What is the WLTP?

It’s a much more rigorous and complex version of the New European Driving Cycle (NEDC) tests, which have been in use (with developments) since 1992 and were widely recognised as being out of date. 

Like the NEDC, the WLTP test is conducted in a laboratory to ensure it’s consistently repeatable, but the challenges the car goes through are much more representative of modern, real-world driving.

For instance, the test is 30 minutes long rather than 20, and 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).

It’s important to note that this test isn’t designed to tell car buyers what mpg they will get from a specific vehicle, the purpose is to make it easier for them the compare the fuel economy of different models. Factors such as traffic, the use of heating and air conditioning and how many people are travelling in car will affect the real MPG a car achieves. 

What Car?'s own Real MPG testing goes a step further, to give an even better indication of the economy you can expert in real-world driving.

What data is available?

There are fuel/energy consumption figures for every model variant of new car on sale, plus driving range stats for plug-in hybrids and electric cars.

For petrol, diesel and hybrid cars fuel consumption figures are based on four different driving conditions: city driving (maximum 35mph); town driving (maximum 50mph); rural driving (maximum 60mph); and motorway (maximum 81mph). There's also an overall fuel economy figure calculated from an average of the four different tests.

For plug-in hybrids, which have an engine and an electric motor that can be charged up via batteries, the mpg figures are based on results when the engine is running and the batteries are depleted. There is also be a weighted combined figure for the vehicle operating on both the battery and engine on a long journey.

Electric cars only have one consumption figure – for electricity used on the combined cycle.

Two driving range figures will be available for plug-in hybrid and pure electric cars: a city range based on the city and town tests, and a combined range based on all four test results.

The aim of providing more data is to help consumers understand which powertrain will best suit their driving needs.

There are a range of fuel economy and CO2 figures for each car model because these figures will vary if a car is fitted with options that affect its weight or aerodynamics.

What is the RDE test?

The WLTP test is 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 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 old NEDC tests was that it was very hard to match the results in the real world.

The current test procedures are more representative. However, while the MPG results are 22% lower than before, on average, 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?

The WLTP test data will be the most useful source of official information; efficiency/energy use labels displayed on new cars show fuel and electricity consumption figures and the driving ranges for plug-in hybrids and electric vehicles.

The WLTP figures are particularly important if you run a company car or van because they play a part in setting the fleet car tax rate you pay

Why were the new emissions tests introduced?

Lawmakers and car manufacturers worked 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.

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

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

Our feature outlines details on our test processes, which have been modelled and carried out with input from test experts at Millbrook.

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 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 Deals service can help you find a great deal.

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