New Vehicle Emissions Tests

New Vehicle Emissions Tests

The emissions scandal eroded public trust in the emissions and miles per gallon figures manufacturers were supplying. The new Worldwide Harmonised Light Vehicle Test aims to provide figures that are more realistic in day to day driving.

What was wrong with the previous emissions test?

The original test for emissions was the New European Driving Cycle (NEDC). It consisted of a laboratory based test using a rolling road. This testing situation allowed for conditions to be kept favourable, no tests with extra loads were required, the test didn't involve performance up and down hills, or take into consideration the effect any extra options had on emissions. It was found the laboratory conditions of the NEDC produced results that could not be replicated on the road.

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The NEDC test also made it possible for manufacturers to use 'defeat devices'. These devices could detect the conditions that signal testing on a rolling road and lower the power level the engine was running at. As soon as normal driving conditions were detected the engine would return to full power. This practice improved emissions and mpg results within the test conditions, but would never be achievable in a real driving situation.

To close the gap between results produced in test conditions and those found in day to day driving a new emissions test was created, the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

The Worldwide Harmonised Light Vehicle Test Procedure

The WLTP is a new tougher emissions test which includes laboratory testing and an on the road real life driving test.

The laboratory portion of the test has been redesigned to better reflect on the road driving conditions. These changes include; an increase in test duration, a longer distance covered, 3 different test cycles depending on the power to weight class of the vehicle, the effect of optional equipment and a range of temperatures to be used during testing from 14°C to 23°C.

Read more: What are the Benefits of Euro 6?

The WLTP recognises that even with these changes, a laboratory test will still never truly produce the same results as driving on a real road. To address this the WLTP also includes a Road Driving Emissions Test (RDE).

When does the WLTP come into effect?

September 2017

The WLTP is already in use. It officially replaced the NEDC in 2017. All new to the market cars were required to use the WLTP test from this date. This means there are vehicles on sale which display both a WLTP value for emissions and a NEDC value.

September 2018

From September 2018 the WLTP will apply to all new registrations. An exception to this is any remaining end of stock cars that underwent the NEDC test. These vehicles can display NEDC figures and be sold up until September 2019.

January 2019

At the start of 2019 all vehicles should be displaying WLTP Co2 values.

Differences between the NEDC test and the WLTP test

  NEDC WLTP - laboratory test
Length of test 20 minutes 30 minutes
Distance 6.8 miles 14.4 miles
Test temperature 20-30°c 14-23°c
Test speed average 21mph maximum 75mph average 28.9mph maximum 81.3mph
Driving phases 2 phases, 66% urban, 34% non-urban 4 phases, 52% urban, 48% non-urban
 Optional equipment  Effect on emissions not considered  Effect on emisisons considered 

The Road Driving Emissions Test

The aim of the RDE test is to ensure the performance measured in the laboratory testing in the WLTP test can be matched on the road. The RDE test is not mandatory until 2019, but manufacturers can choose to put their vehicle through this test.

The test will last for between 1.5 and 2 hours and will cover at least 29.92 miles. During the RDE, testers will be required to drive on urban and rural roads as well as on a motorway for at least 5 minutes, at a speed of at least 62.13mph. The vehicle will also be tested in extreme conditions that are unlikely to be encountered in day to day life, such as carrying a heavy load up a steep hill at high speed in very low temperatures.

How does the RDE test emissions?

The RDE test will measure carbon dioxide, nitrogen oxide and carbon monoxide. To do this a Portable Emissions Measurement System (PEMS) is fitted to the vehicle and can measure emissions as the vehicle is driven around.

When comparing the reading from the PEMS machine to the Euro 6 emissions standards a margin of error will be allowed, which is being called a conformity factor. This has been deemed necessary for two main reasons;

  1. The PEMS machine will not produce reliable, repeatable results. It is incredibly sensitive and will be affected by changes in temperature and altitude which can not be kept standardised over all RDE tests as different roads will be used in different driving conditions.
  2. There is no set standard of PEMS machine to be used. It is likely that different PEMS machine from different manufacturers would produce different results even if all the other variables were kept the same.

The conformity factor produces a 'limit not to exceed' and will be reduced in the coming years with the use of RDE 1 and RDE 2

What are RDE 1 and RDE 2?

RDE 1 and 2 are the same RDE test. The difference is the conformity factor that will be applied to results to decide whether a vehicle is Euro 6 compliant or not.


RDE 1 will be applied until January 2020. It will allow an emission reading of 2.1 times the Euro 6 Nox limit to be produced and still be Euro 6 compliant.


RDE 2 will apply from January 2020 onwards for all new to market cars and from January 2021 for all new registrations. It allows for a much smaller conformity factor of 1.5, so any vehicle emitting Nox emissions 1.5 times the Euro 6 level will still be compliant.

Read more: New Road Tax Rules Explained!

Tax implications of the WLTP from April 2018

  • Any diesel vehicle which does not have a RDE 2 pass certificate will be treated as though it is in the tax band above for the purposes of the first year Vehicle Excise Duty rate.
  • For company car drivers, the 3% diesel surcharge applied will increase to 4% for diesel vehicles which do not have a RDE 2 pass certificate.

Overall, the WLTP and RDE tests aim to provide the public with a more realistic measure of emissions and mpg to help consumers make a more informed choice.

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