What is the Real Driving Emissions (RDE) test?

The new test that's raising the cost of diesel cars: Real Driving Emissions (RDE)

BuyaCar team
Mar 28, 2018

The name might remind you of Star Wars’ endearing robot R2D2, but there’s not a lot of charm about RDE2: a little-known emissions standard that’s about to push up new car diesel prices.

It stands for Real Driving Emissions Step 2, and imposes stricter limits for toxic compounds in exhaust fumes.

The measure was thrust into the spotlight in the November 2017 budget when the Chancellor declared that any new diesel cars registered from 1 April 2018 that do not meet the standard will move into a higher road tax band, although only in the vehicle’s first year.

Anyone buying a new diesel car is likely to have to pay more because new cars won’t have to meet the standard until January 2020 at the earliest, so manufacturers have not yet introduced compliant cars.

The RDE test will not have any impact on cars that are already on the road; used car buyers will be unaffected too.



What is the RDE test?

Every type of car has to be tested in a laboratory to ensure that they comply with emissions standards.

But it’s been known for several years that the test wasn’t a good indication of real-world driving because it was carried out at a slow average speed and only required gentle acceleration.

It was also easy to cheat, as was shown during the dieselgate scandal, when Volkswagen was found to have programmed its cars to switch on emissions controls only during the laboratory test.

That’s why the Real Driving emissions test was introduced in September 2017. For the first time, it measures vehicle emissions during real-world driving and is used to check that cars are as clean on the road as they are in a laboratory.


What are RDE steps 1 and 2?

At the moment, regulators have allowed an introductory period (Step 1), where vehicles can produce twice as many harmful emissions as in the laboratory and still comply with the RDE standard to take into account differences between the test and real-world driving.

But that limit will be lowered from January 2020, when the new RDE2 standard will come into place. A smaller difference between lab test and RDE test will be allowed, to take into account for inaccuracies in the testing equipment.

Any brand new car that’s launched after January 2020 will have to meet the RDE2 standard. Every new car will have to meet it by January 2021.

Any diesel car that meets the RDE2 standard early will be exempt from the higher level of road tax that’s being imposed on new diesel cars from April 1, so tax will cost no more than an equivalent petrol car.


How does the RDE test affect diesel cars?

Every new car sold from September 2018 will have to met the RDE Step 1 test. If they don’t, then manufacturers won’t be able to sell them.

The test doesn’t apply to cars that are already on the road, so you shouldn’t be concerned that this will affect you.

In coming years, new diesel car buyers may be able to save money by getting a car that meets the tougher RDE Step 2 standard (none do at the moment). These vehicles won’t be subject to the higher rate of diesel car tax that will add £310 to the cost of first-year tax for many models, and it’s likely that this will be reflected in the price of the car.


How does the RDE test work?

A bulky piece of testing equipment, called a Portable Emission Measuring System (PEMS), is attached to the car’s exhaust pipe, as shown above.

This measures the level of nitrogen oxides (NOx) and tiny pieces of soot, known as particulates, which have been linked to respiratory diseases and early deaths.

Cars are driven on urban and rural roads, as well as motorways, with routes including hills to try and reflect the range of road conditions.

Emissions figures are then calculated using software that makes adjustments for weather and traffic conditions, so that each vehicle’s result should be based on the same standard.


How effective will the RDE test be?

Independent real-world tests have found that some diesel cars produce NOx at twelve times the limit allowed in official laboratory procedures. Under the RDE test, these vehicles will not be legal to sell as new cars (used cars are unaffected).

In addition, the introduction of the RDE test came at the same time as a new laboratory test called WLTP (Worldwide Harmonised Light Vehicle Test).

This tests cars at higher speeds, under harder acceleration, and while using equipment such as the radio and air conditioning, to provide a more accurate measure of emissions (as well as fuel consumption).

It’s expected that cars will need to be cleaner to meet the WLTP test - and then go on to prove that they can repeat that performance on public roads with the RDE test.

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