Emissions explained

Blog post posted on 07/09/17 |

On 1 September two important, and complementary, changes to car fuel economy and exhaust emission figures took effect. The Worldwide Harmonised Light Vehicle Test Procedure, or WLTP for short, and the Real Driving Emissions test, RDE, were both introduced.

WLTP deals with the “official” mpg and CO2 figures and is intended to make them more realistic. Motorists have complained for years that cars do not achieve the advertised fuel economy and that was largely because the old way of testing them (known as the NEDC) did not replicate modern driving. Using more fuel also means the cars produced more CO2, making it harder to meet our national carbon reduction targets to reduce the danger of global warming.

Although the new test will give a figure closer to what can be achieved in normal driving, it should still be seen as a way of comparing different cars’ performance, rather than a guarantee of what fuel economy the car will return.

The new test applies to any new model of car launched from the 1st September onwards. Existing models will still show the old mpg and CO2 figures until September 2018 when all new cars on sale will have to show the new test results.

In general, the mpg will be lower and the CO2 emitted will be higher than under the old test.  However, somewhat confusingly, taxes based on CO2 such as the first year “showroom” tax and company car tax will still be based on the old, NEDC figure for at least a couple of years. (The NEDC figure will continue to be calculated for all cars until about 2020)

The second change, introduction of RDE, addresses emissions such as NO2 and soot, which are health hazards.  These pollutants should have been limited by increasingly tough mandatory standards known as Euro 1, Euro 2 and so on. We are now at Euro 6, but pollution levels in towns and cities have not fallen the way the standards should have achieved because, once again, vehicles do not perform in real driving conditions as well as they do in a laboratory under test conditions.  The widely publicised case of VW also showed that some manufacturers were prepared to bend the rules to achieve the required figures.

From now on, cars will not only have to pass an emissions tests in the lab, but also on a 90 minute drive on real roads.  Portable emissions testing equipment will be fitted to cars driven on town, country and motorway roads and the vehicles must not exceed strict pollution limits. Ensuring the cars genuinely produce less pollution will help to clean up our towns and cities the way the Euro regulations were intended to. As with WLTP, the new test applies to brand new models from 1st September, but will only come into force for all new cars from September 2019.

So, WLTP will give a more realistic guide to fuel economy and help drive down the global warming CO2 emissions from vehicles while RDE will reduce the health harming toxic pollutants.

These changes are intended to make driving less damaging to people and our planet and IAM Roadsmart, through our work with the Low Carbon Vehicle Partnership, have been closely involved with their introduction. For more detailed information on the new mpg tests, follow this link:


By Tim Shallcross, IAM RoadSmart head of technical policy and advice