A study published Sept. 3, 2026 by the International Council on Clean Transportation found that plug-in hybrid electric vehicles registered in the European Union are emitting far more carbon dioxide on the road than their official sticker figures promise, and the gap is widening rather than closing even as automakers advertise longer electric-only ranges. Real-world CO2 emissions from cars registered in 2023 ran at 4.6 times their official type-approval value, up from 3.5 times for cars registered in 2021. The research draws on onboard fuel-monitoring data from roughly 920,000 plug-in hybrids sold across Europe between 2021 and 2023.
How the ICCT Measured the Gap
The organization, the same research group behind the 2015 “Dieselgate” revelations about Volkswagen’s diesel emissions, compared each vehicle’s official type-approval CO2 rating against fuel-consumption data logged by the car’s own onboard monitor during real-world driving. That method sidesteps the laboratory test cycle entirely and instead captures how owners actually use their vehicles, including how often they plug in versus rely on the combustion engine. Applying it to nearly a million cars let researchers track how the gap between sticker and street changed year by year across the 2021-2023 registration cohort. The official type-approval figures are themselves built on an assumption about how many kilometers a given vehicle will travel on battery power versus on its combustion engine, and the ICCT’s central finding is that assumption diverges further from reality with each newer model year, describing what it called a “substantial and fast-growing gap between real-world and type-approval CO2 emissions” for the vehicles it tracked.
Why the EU’s Utility Factor Formula Overstates Electric Driving
The widening discrepancy traces back to the European Union’s utility factor formula, the calculation regulators use to estimate how much of a plug-in hybrid’s mileage will be driven on electric power alone before assigning it an official CO2 rating. That formula assumes electric driving accounts for roughly 80% of a plug-in hybrid’s mileage, and it assumes the share rises further as electric-only range increases. The ICCT’s data put the real-world figure at only about 21% for 2022, and found no evidence that longer ranges pushed that number up. Jan Dornoff, the ICCT’s research lead and the study’s author, told Automotive World that plug-in hybrids registered in 2023 “emitted almost 5 times more CO2 on the road than their official values suggest, and the discrepancy has grown over the years,” adding that the mismatch gives the vehicles “an unjustified advantage over combustion engine vehicles” when automakers’ fleetwide CO2 compliance is calculated.
What the Mismatch Means for Buyers and Emissions Targets
Because official CO2 figures also underpin the fuel-cost estimates shown to shoppers, the gap misleads buyers about how much they will actually spend on fuel, not just how much the vehicle pollutes. It also matters for regulators: plug-in hybrids’ artificially low official ratings let them count disproportionately toward automakers’ compliance with the EU’s fleet-wide CO2 reduction targets, even though the emissions reductions those ratings imply are not being achieved on the road. The mismatch further skews the eligibility of plug-in hybrids for the tax breaks and purchase incentives that several European governments still tie to a vehicle’s official CO2 rating, effectively rewarding cars for emissions performance they do not deliver in daily use. The European Commission has already scheduled a correction to the utility factor formula for 2027, intended to bring the assumed share of electric driving closer to reality.
A Market Still Growing Despite the Findings
Plug-in hybrids held about 10% of new European vehicle registrations between January and July 2026, essentially flat compared with 9% over the same period in 2025, while fully electric vehicles reached 22% of registrations. Chinese manufacturers accounted for 28% of the plug-in hybrid segment, up 60% year over year, with BYD drawing 60% of its own European registrations from plug-in hybrid models, compared with 17% for Mercedes-Benz, 13% for BMW and 12% for Volkswagen. Mercedes-Benz was Europe’s largest plug-in hybrid manufacturer across the 2021-2023 period the study covers, and the ICCT found its vehicles carried the widest gap between official and real-world emissions of any manufacturer measured.
The Debate Over Plug-In Hybrids’ Role in the EU’s Combustion Timeline
The findings arrive as automakers, particularly in Germany, are pushing the EU to preserve a larger role for plug-in hybrids in its road map for decarbonizing new-car sales. The EU dropped its earlier plan to prohibit registrations of new combustion-engine vehicles in 2035, replacing it with a requirement that manufacturers cut fleet CO2 levels 90% from 2021 levels while offsetting the remaining 10%. Germany and allied governments are now seeking further loosening of those rules to give hybrid vehicles more room, according to wire reporting on the ongoing negotiations. The ICCT is urging the European Commission not to delay the 2027 utility factor correction and to base any further formula updates directly on real-world monitoring data rather than laboratory assumptions.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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