A new study from the International Council on Clean Transportation (ICCT) has found that plug-in hybrid vehicles (PHEVs) in Europe emit substantially more carbon dioxide in real-world driving than indicated by official certification figures, with the gap widening in recent years.
According to the research, real-world emissions from plug-in hybrids were, on average, 400% higher than official values in 2023, equivalent to approximately five times the certified level. The discrepancy increased from 265% in 2021, which the ICCT attributed largely to an overestimation of how frequently drivers operate these vehicles in electric mode.
Gap Between Official and Real-World Emissions Widens
Plug-in hybrids accounted for around 9% of new vehicle sales in Europe in 2025. These vehicles combine a battery-powered electric motor with a gasoline or diesel engine, allowing drivers to switch between electric and combustion power.
While manufacturers promote plug-in hybrids as a lower-emission alternative to conventional vehicles, the ICCT said multiple studies have raised concerns about the accuracy of their official fuel consumption and emissions ratings.
“Plug-in hybrids consume far more fuel on the road than official figures suggest. This study reinforces earlier findings, including our own,” said Sonsoles Díaz, Senior Researcher at the ICCT.
“Unless regulators address this flaw, carmakers will continue to report emissions that are far lower than those produced in real-world driving.”
Mercedes Recorded Largest Emissions Gap
The study analyzed approximately 8 million vehicles, including gasoline, diesel, hybrid and plug-in hybrid models.
Among major manufacturers, Mercedes, identified by the ICCT as the largest seller of plug-in hybrids between 2021 and 2023, recorded the widest discrepancy between certified and real-world emissions. The gap increased from 329% in 2021 to 614% in 2023, averaging 452% over the three-year period.
The ICCT estimated that underreported emissions from plug-in hybrids registered between 2021 and 2025 amount to roughly 100 million metric tons of CO2. According to the organization, those emissions were not reflected in the European Union’s vehicle emissions reduction targets.
EU Rules Updated but Further Debate Continues
The European Commission revised its “utility factor” methodology in 2025 to better reflect real-world plug-in hybrid usage patterns, with a further adjustment scheduled for 2027.
However, the future treatment of plug-in hybrid emissions remains under discussion as the European Parliament reviews the EU’s vehicle CO2 reduction framework. An early draft proposal reportedly calls for freezing future revisions to the emissions calculation methodology.
“Even with the 2025 correction, emissions on the road are not accurately estimated,” said Peter Mock, Europe Director at the ICCT.
“And the worrying trend is that new models are not emitting less but more. Even if carmakers claim that consumers drive more kilometers in electric mode, the reality is something else.”
Battery Electric Vehicles Drive Most Emissions Reductions
The study found that emissions reporting discrepancies exist across all vehicle powertrains, although the gap is significantly smaller for conventional vehicles.
According to the ICCT, gasoline and diesel vehicles, including full and mild hybrids, recorded an average difference of about 20% between certified and real-world emissions in 2023, compared with approximately 400% for plug-in hybrids.
Between 2018 and 2023, official average CO2 values for new passenger vehicles fell by 28%, while real-world emissions declined by only 15%. The organization said battery electric vehicles accounted for most of the reduction, while real-world emissions from combustion-engine vehicles declined by only about 1% during the same period.
“The gap of plug-in hybrids is shockingly high, but this should not distract from the fact that the gap of conventional vehicles, which still make up the majority of vehicle sales in the EU, is also considerably high at 20%,” said Jan Dornoff, ICCT Research Lead and co-author of the study.
“As a consequence, we didn’t see any notable reductions in the real-world CO2 emissions of vehicles with a combustion engine over the past years.”