Manufacturers’ projections of plug-in hybrid emissions don’t match up with the way people actually drive, find reports that show plug-in hybrid vehicles are powered by fossil fuels more often than assumed.

Plug-in hybrid electric vehicles (PHEVs) pair a battery-powered electric motor with a gasoline engine, allowing them to operate using electricity, gasoline, or both. They don’t deliver the same emissions-reducing benefits as fully electric vehicles, but amid reliability concerns among drivers who haven’t experienced EVs first hand, advocates have treated PHEVs as a good compromise.

In some regions with rough terrain or limited access to EV charging, PHEVs are the only realistic option to begin decarbonizing vehicles. Governments also rely on PHEV emissions reductions to make regulations and short-term targets appear more attainable at a time when limited EV manufacturing capacity and charging infrastructure constrain wider adoption.

But new research from Transport & Environment (T&E) and the European Commission has found that real-world PHEV emissions are not as low as expected. While official estimates assume emissions from PHEVs should be about 75% lower than their fossil fuel counterparts among European drivers, T&E finds they are actually only 19% lower on average.

The gap mostly comes down to “flawed assumptions” about how much driving is done in electric driving mode.

PHEVs have three modes for driving: solely on the charge of the electric motor, mostly on the charge of the combustion engine, or fully on the charge of the combustion engine while also repowering the battery. The amount of driving in different modes determines how efficiently the vehicle operates. Official projections have assumed that a PHEV with a 70-kilometre range would drive about 80% of the time in electric mode, delivering transportation with significantly fewer emissions than fossil fuel cars. But that number is set to be corrected for 2025/26 and 2027/28 projections, down to 54% and 34% of the time, respectively.

Moreover, in real-life driving, people may not charge a car’s electric motor often enough to maximize the distance driven in electric mode, T&E says. PHEV electric motors are also comparatively weak, so they don’t accommodate drivers at higher speeds. And the combustion engine could automatically kick in if drivers accelerate too quickly, even if the speed and battery charge would have otherwise kept the vehicle in electric mode.

T&E’s analysis shows that PHEVs are driven on the electric motor 41% of the time, on average, including combined mode with the combustion engine also working. Only 27% of driving distance is done with purely electric driving. This means emissions from PHEVs can be as much as five times higher than expected.

Findings like those in T&E’s report have important implications for regulations to cut transport sector emissions. Many governments, like those in the European Union, include emissions cuts from electrifying transportation as part of their decarbonization strategies. But if PHEV emissions are higher than accounted for, including them in automaker mandates is not achieving the expected climate progress.

Canadian automakers also relied on PHEVs to meet zero-emissions vehicle targets as part of the federal EV mandate that has since been delayed for one year by the new government led by Prime Minister Mark Carney. (The studies in the EU do not evaluate PHEV driving behaviour in Canada, though other research by the International Council on Clean Transportation includes PHEV data from the U.S.)

Adam Thorn, director of the Pembina Institute’s transportation program, told CBC News the findings are an “educational opportunity” that will “really make sure that those who purchase those vehicles are aware of that opportunity to charge the vehicles overnight, for example, [and] recognize that they’re really not going to get the full fuel savings if they don’t do so.”

“We really do think it’s a key policy to drive down emissions,” said Thorn, despite the T&E findings.