Beneath the headlines about plug-in hybrids, a deep-dive into the real-world data suggests they’re making a useful contribution to cutting CO2, as editor at large Alex Grant explains.

Plug-in hybrids are the UK’s fastest-growing powertrain technology

Almost exactly 10 years after the Mitsubishi Outlander PHEV launched in Europe, the technology that car helped to popularise is still making headlines.

Plug-in hybrids (PHEVs) are the UK’s fastest-growing powertrain technology, with a 7.8% market share and 31.4% increase in registrations year to date, according to the SMMT.

They’re also facing some familiar criticism – a new study by the European Environment Agency (EEA) has shown a sizeable gap between published and real-world efficiency figures. Colour me shocked.

Although the statement isn’t novel, it’s rooted in the largest dataset to date. Since January 2021, every new fuel-burning car and small van sold in the EU has had to include an onboard fuel consumption monitoring (OBCFM) device. It’s a system that logs real-world fuel and electricity usage and, at a glance, the results are damning.

CO2 emissions across the PHEV fleet (almost 124,000 vehicles) were 250% higher than under the WLTP test cycle, which appears to undermine their green credentials. Or does it?

Anyone who is familiar with PHEVs knows their WLTP figures are arbitrary – and that real-world efficiency is near infinitely variable depending on where they are driven and how often they are charged. A diligent local commuter might well beat the WLTP figure, whereas a colleague covering high mileage in the same car and never plugging in might come in at 80% less than advertised.

Most drivers should come somewhere in the middle, using less fuel than they would in a traditional petrol or diesel car – and that’s the important point.

The EEA’s data shows this is happening – tailpipe CO2 emissions for petrol and diesel PHEVs are 24% and 15% lower, respectively, than their non-hybrid counterparts – despite being 39% and 31% heavier. Audi and BMW’s petrol PHEVs emit 20% less CO2 than their broadly comparable diesel line-up, while Toyota’s results are also worthy of note. Its PHEV data comes from a fleet of 40 RAV4s, which are averaging 69mpg and 93g/km in real-world use. Yes, fuel consumption and CO2 emissions are four times higher than WLTP, but both are respectable for a 306hp, four-wheel drive SUV.

Dig into the raw data spreadsheet (individual vehicles, instead of grouped by manufacturer) and the results become even more interesting. As the EEA points out, there are gaps. Almost 3,500 PHEVs hadn’t reported their EV-only mileage, while another 11,000 had less than 1,000 miles on the clock, so I filtered them out.

The remainder are returning 47.3mpg as a lifetime average, covering 27% of their total mileage on battery power and 33.1mpg with the engine running. That compares to non-hybrid petrol cars at 35.8mpg and diesels at 41.1mpg. Most (97%) of my filtered sample had been plugged in at least once, 83% had received at least 100kWh of mains electricity and drivers were actively using the engine to charge the battery for 4% of the total mileage.

Even PHEVs covering more than 20,000 miles are averaging 40.4mpg and around 2,500 electric miles – a lot of which will be in urban areas, where curbing air pollution is most important. Almost one in 10 PHEVs has a lifetime average efficiency figure exceeding 100mpg.

PHEVs have always been a stepping stone – regulations and improving BEV performance will make them obsolete (as new cars) in Europe by 2035. In the meantime, the best available data shows drivers are actively using the technology and reducing their CO2 emissions.

Ten years after the Outlander’s bumpy landing, perhaps it’s time to stop pretending we can reflect that complexity in a single laboratory-derived figure and have a more useful discussion about the pros and cons.