The WLTP enigma

Is there something strange happening with WLTP?

The Worldwide Harmonised Light Vehicle Test Procedure (WLTP) is the new laboratory certification test for light duty vehicles in Europe. In particular, it is used for the fuel economy labelling of vehicles and the carbon dioxide (CO2) results will be used in labelling and manufacturer fleet average CO2 calculations. Missing the fleet average targets could trigger significant fines for manufacturers.


WLTP includes a more dynamic test cycle compared to the old New European Driving Cycle (NEDC), although it remains entirely in the laboratory. There is also a new legal framework, which removes some the loopholes and grey areas previously present, which were believed to contribute to the large gaps between certification values and the real world. Therefore, through the combination of the more dynamic cycle and fewer loopholes, the expectation was that WLTP results would see smaller discrepancies with real-world performance.

All cars sold from September 2018 must be WLTP certified, but the fleet average targets are still judged relative to NEDC values. Therefore, a translation mechanism has been created to derive NEDC-equivalent values from the WLTP test. The Joint Research Centre of the EU expressed concern about manipulation of this mechanism in 2018, and tightened the system as a result.

Is WLTP now functioning as intended? How do we judge?

Fortunately, Emissions Analytics has been conducting the consistent, standard EQUA Index test since 2011, bridging both systems. Over 1200 vehicles have been tested so far in this way. Using this data, we are able to compare the old NEDC figures, new NEDC figures and WLTP values to this consistent reference point to understand what has been happening.

There is a vital need to bring clarity, as the WLTP system is complex. Even if data is made freely available, it is hard to understand and analyse. Often, however, the CO2 data is not easily and comprehensively available. The following analysis relies on data quoted by manufacturers in their vehicle registration documents, except where it is demonstrably inaccurate.

The table below show NEDC values derived under the old system (NEDCNEDC), NEDC values under the new system (NEDCWLTP) and EQUA Index results for vehicles certified under NEDC and WLTP respectively (EQUANEDC and EQUAWLTP). NEDCWLTP results are primarily derived by processing the data from the WLTP test through a software package (CO2MPAS) provided by the EU. The process is complex and, in fact, the NEDCWLTPvalues could be modelled using CO2MPAS, manufacturer declared values or from actual dynamometer tests – therefore, the determination of NEDC-equivalent values under the new system is potentially open to manipulation as its complexity does not assist transparency.

The table summarises the average CO2 emissions for all pre- and post-WLTP vehicles tested up to 3 litres in size. In total, 25 WLTP cars have been tested so far that have a full set of publicly-available data.


To start, it is worth noting that the real-world emissions on the EQUA Index sit well above the current 130 g/km fleet average CO2 target, let alone the 2021 95 g/km 2021 target. But most striking are the differences in trends between diesel and gasoline vehicles, as shown in the table below.


Across all the Euro 6 cars tested prior to the introduction of WLTP, the average CO2 gap was 32%. Cars certified to WLTP over the last one-and-half years show a smaller gap of 13%, as expected. However, below these intuitive headlines are some perplexing trends. For example, the gap between the EQUA Index and the NEDC for diesels has shrunk, while it has expanded for gasoline vehicles – to the point that there is no systematic gap between the EQUA Index and WLTP for diesels, but there is a 24% gap for gasoline.

To verify that this is not an artefact of the vehicles tested, comparing the average CO2 for EQUA results under the NEDC compared to EQUA Index results under the WLTP showed a difference of just 2%. Furthermore, the average engine sizes of the two cohorts were considered, and were broadly similar:


Therefore, it is clear that there has been a significant change in the relative certification values of diesels and gasoline, despite no change in the real-world performance as measured by the EQUA Index.

So, what can we conclude about the health of the WLTP system?

The table below summarises the findings. The comparison of WLTP to NEDCWLTP results is intuitive and in line with EU forecasts. The health of the translation system is primarily being policed by looking at this difference because of the concern that OEMs were simultaneously inflating WLTP values and suppressing NEDC values, to create the biggest gap, so that future WLTP-based fleet average CO2 reductions were based off an inflated start point. The results below suggest this issue is not occurring on a significant scale.

The difference between NEDCWLTP and NEDCNEDC should be positive, as the new type approval framework removes loopholes – but there is a significant negative gap for gasoline vehicles. In other words, CO2 values have fallen despite the test getting tougher. Comparing WLTP to the average NEDC values on under the old regime shows a big rise in CO2 for diesels, but virtually no change for gasoline vehicles.


A potential explanation for these movements is that focus and expertise have been put into optimising gasoline vehicles on the new WLTP cycle, driving the official CO2 down despite this not translating into better real-world performance. The increase in official diesel CO2emissions may be partly explained by the larger average engine size of engines so far tested.

If this turns out to be true, it would give further credence to the argument for including on-road testing in the official CO2 certification process, to limit the ability to optimise to the test. In the meantime, the use of WLTP results for fuel economy labelling will be hardly more useful to consumers that the old, discredited NEDC system it replaced

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