Recent plug-in hybrids can emit far more CO₂ on the road than their type-approval figures indicate.
The ICCT studied data from around 920,000 cars registered in Europe between 2021 and 2023. The data came from onboard systems recording actual fuel and energy consumption. For vehicles registered in 2023, the average emissions observed reached 4.6 times their official value.
Type 2 connector plugged into the charging socket of a BMW 330e iPerformance.
Credit: D-Kuru/Wikimedia Commons — CC BY-SA 4.0.
A plug-in hybrid combines a rechargeable battery with a petrol or diesel engine. Some journeys can therefore be completed primarily using electricity. When the battery is no longer sufficient, the combustion engine partially or fully takes over. The result then depends heavily on how often the vehicle is recharged.
To calculate official emissions, type approval must estimate what share of the kilometres will be driven primarily using the battery. For the vehicles studied, this share was estimated at around 80%. Data collected on the roads indicate an actual proportion close to 20%. The combustion engine therefore operates far more often than assumed in the calculation.
The gap widened between 2021 and 2023. Average real-world emissions rose from around 130 to 134 g of CO₂/km. At the same time, official figures fell from 37 to 29 g/km. Type-approval figures therefore improved without a comparable reduction in emissions observed in actual traffic.
One might think that a longer electric range automatically means more kilometres driven without petrol. Recent models do indeed have batteries that allow them to travel greater distances. Yet the ICCT finds that the share actually driven primarily on electricity did not increase with this range. On the contrary, it decreased in the data studied.
In practice, two people driving the same model can achieve very different fuel consumption figures. A driver who recharges frequently and makes short trips will benefit more from the battery. If the car is rarely plugged in, its combustion engine will run much more often. A single official figure therefore struggles to reflect all these uses.
These calculations also have consequences for manufacturers. The European Union uses type-approved emissions to track the average of new cars sold by each brand. An initial change to the method has applied since 2025. A second correction is planned for 2027 to better account for the observed use of plug-in hybrids.
The ICCT now proposes regularly revising this method using data recorded by vehicles in circulation. The assumptions used for future type approvals could thus keep pace with real changes in charging and driving habits.