Since September 2017 all new car types have been tested under WLTP, the World Light Testing Protocol: designed to eradicate most of the weaknesses of the previous test regime NEDC, and present more realistic figures to consumers. By September 2018 any new car for sale should have been tested under WLTP, other than a very small percentage of old models. The test deadlines were imposed on all EU members by Regulation 2017/1151 whose formulators perhaps envisaged that the test data would be made public in line with the tests. However the UK government has instead required that fuel consumption figures must be published from 19th January 2019 and that emission figures including CO2 should be used for taxation purposes for cars registered after 5th Apil 2020.

The Directive did lay out an adjustment mechanism for fleet targets since these were set on the basis of more lenient NEDC testing, and this is covered by a calculation programme known as Co2mpas which converts WLTP results on ICE engines back into NEDC equivalents.. There is a certain amount of iteration and adjustment for several factors, but if a manufacturer uses it too aggressively, their CO2 figures will be subject to an adjustment factor at the end of each year. The tool will adjust for the higher speed, acceleration and mass adjustments used in WLTP testing, but don't expect it to adjust for cycle beating and weaknesses of the NEDC test regime.

Meanwhile the car industry has latched on to the potential of the Co2mpas tool as a way of protecting drivers from sudden increases in CO2 based taxation, and got support from an EU recommendation that national taxation schemes should be neutral in terms of the move to WLTP testing for a transitional period.  So when a car is tested under WLTP, the results in most cases are then translated into an NEDC eqiuvalent. Both the WLTP and NEDC equivalent figures appear on the vehicle test certificate, so drivers will want to check that the lower CO2 figure is used by their payroll department when filling in the P11D or P46(car).

 Those ordering new cars now are wondering why the emission figures have jumped up in most cases, even though the vehicles have the same description and NEDC equivalent figures. There are two main reasons, though the contribution of each factor can vary widely:

  1. The car itself may have changed to meet tougher air quality standards. RDE testing also started at the same time as WLTP, so vehicles may need additional equipment to pass on-board NOx tests which have "must not exceed" targets whilst driving on real roads. There is no strong engineering reason why this should force up CO2 emissions, but the extra technology needed to burn up or capture NOx emissions can reduce performance.
  2. WLTP testing is more comprehensive than NEDC and has fewer loopholes. The Co2mpas tool will only adjust for official differences and not the flexibilities that manufacturers learned to exploit over decades of NEDC testing.

Using the new Mazda6 as an example, we have attempted to illustrate what is going on. The arrows indicate the important sequence of testing which moves from NEDC to WLTP and then back to NEDC equivalent. You cannot just jump from NEDC to NEDC equivalent. How close you can get depends on the effectiveness of the Co2mpas tool in translating a WLTP figure back to NEDC. In this example it appears that the tool is good at correlating the diesel figures but poor when it comes to the petrol engines. Whether this pattern is repeated on other manufacturers and engines we will have to wait and see. 

Perhaps it would be a good strategy for drivers to seek out company cars where the Co2mpas tool is aggressively applied, but we doubt this information will be easy to find. Instead drivers need to look at the information available and pin down their supplier to make sure there are no unwelcome surprises. Cars tested under NEDC are still available in some ranges, and will keep those CO2 figures for their lifetime.