The European Commission’s 2024 vehicle emission’s report highlights a discrepancy between the official CO₂ emissions reported by car manufacturers and the actual emissions observed on the roads. But what is causing this gap? Understanding why vehicles emit more than homologation estimates is fundamental for developing effective policies to achieve the European Green Deal’s goal of reducing transport sector emissions by 90% by 2050.
Since 2019, all new light-duty vehicles that are to be registered in the EU must comply with the Worldwide harmonised Light vehicle Test Procedure (WLTP) standards. These standards estimate CO₂ emissions under real-world driving conditions by simulating such scenarios in laboratory settings, thereby preventing the sale of vehicles that exceed the established emission limits.
However, the Commission’s analysis shows that real-world CO₂ emissions are significantly higher than WLTP estimates. Petrol cars emit 23.7% more CO₂, while diesel cars emit 18.1% more than the WLTP figures. Although the WLTP has reduced the emissions gap from about 40% under the previous testing standards, there is still a considerable difference between tested and actual emissions.
The discrepancy is even more pronounced for plug-in hybrid electric vehicles (PHEVs), which combine an electric motor with a traditional combustion engine. According to the Commission, plug-in hybrid cars emit, on average, 3.5 times more CO₂ than WLTP values. This large gap suggests that PHEVs are not being driven in electric mode as often as expected, a finding supported by other scientific studies. For example, the International Council on Clean Transportation (ICCT) report estimates that private PHEVs use electric power for only about 45%–49% of their driving, and company vehicles for just 11%–15%. In contrast, the WLTP assumes that PHEVs operate in electric mode 70%–85% of the time.
As a result of this lower-than-expected electric usage, real-world fuel consumption in PHEVs is significantly higher than WLTP projections. Private PHEVs consume around 4.0–4.4 liters per 100 km, and company cars use approximately 7.6–8.4 liters per 100 km, compared to the WLTP’s estimates of 1.6–1.7 liters per 100 km. This substantial gap underscores the need for more accurate emissions testing conditions and better real-world usage strategies to align actual PHEVs performance with regulatory standards.
(Source: European Commission’s report on real-world CO₂ emissions of light-duty vehicles)
Moving forward, the Commission plans to update how vehicles—especially plug-in hybrids—are tested by incorporating more real-world driving patterns. This improvement is essential not only for developing effective policies but also for maintaining public trust in emission reporting and environmental commitments.
(Image by Jacek Dylag on Unsplash)
Researcher in Energy Systems at ISAAC – SUPSI