Plug-in hybrids are supposed to be the best of both worlds—the convenience of a gas-powered car with the climate benefits of a battery electric vehicle. But new data suggests that some official figures severely underestimate the emissions they produce. 

According to new real-world driving data from the European Commission, plug-in hybrids produce roughly 3.5 times the emissions official estimates suggest. The difference is largely linked to driver habits: people tend to charge plug-in hybrids and drive them in electric mode less than expected.