The problem with plug-in hybrids? Their drivers.

According to the new European Commission report, drivers in plug-in hybrid vehicles produce about 139.4 grams of carbon dioxide for every kilometer driven, based on measurements of how much fuel vehicles use over time. On the other hand, official estimates from manufacturers, which are determined using laboratory tests, put emissions at 39.6 grams per kilometer driven.

Some of this gap can be explained by differences between the controlled conditions in a lab and real-world driving. Even conventional combustion-engine vehicles tend to have higher real-world emissions than official estimates suggest, though the gap is roughly 20%, not 200% or more as it is for plug-in hybrids.

The major difference comes down to how drivers tend to use plug-in hybrids. Researchers have noticed the problem in previous studies, some of them using crowdsourced data. 

In one study from the ICCT published in 2022, researchers examined real-world driving habits of people in plug-in hybrids. While the method used to determine official emissions values estimated that drivers use electricity to power vehicles 70% to 85% of the time, the real-world driving data suggested that vehicle owners actually used electric mode for 45% to 49% of their driving. And if vehicles were company-provided cars, the average was only 11% to 15%.

The difference between reality and estimates can be a problem for drivers, who may buy plug-in hybrids expecting climate benefits and gas savings. But if drivers are charging less than expected, the benefits might not be as drastic as promised. Trips taken in a plug-in hybrid cut emissions by only 23% relative to trips in a conventional vehicle, rather than the nearly three-quarters reduction predicted by official estimates, according to the new analysis.

“People need to be realistic about what they face,” Dornoff says. Driving the vehicles in electric mode as much as possible can help maximize the financial and environmental benefits, he adds.