Car makers and their suppliers have defended plug-in hybrid technology after data extracted from cars on the road by the European authorities showed that models on average were emitting more than three times the carbon dioxide shown in official tests.

Sales of plug-in hybrids (PHEVs) grew 34% in the first three months in the UK to 42,492 as more company car drivers took advantage of their tax-friendliness. That in turn helps car makers to drive down their average CO2 emissions to meet legislation and softens their 22% electric car target this year. In Europe, sales were up 7.4%, with the UK now only fractionally behind Germany for the title of the biggest PHEV market in the region.

However, figures collected by the European Commission as part of a wider study into the effectiveness of the WLTP fuel economy test show that plug-in hybrids might not deserve their low-CO2 status, along with the perks that go with that. 

Under new rules that allow European authorities to download data from cars in use, the European Commission found that among the 191,197 PHEVs interrogated, the average fuel economy was just 47.5mpg against the 167mpg claimed through the WLTP test. 

The commission concluded that the “large discrepancy” between figures achieved by PHEVs in the lab test versus the real world “shows that they are charged and driven in electric mode much less than how they were expected to be used”. The – perhaps predictable – assessment was that “assumptions used for calculating the WLTP test result do not hold in real-world conditions”.

BMW, the UK’s third biggest seller of PHEVs in the UK so far this year, pointed to the official tests in its defence of the technology, which requires owners to charge the battery via a plug to gain the full benefit. 

“These tests show clearly that plug-in hybrid technology, when the vehicles are charged regularly as intended, can save significant fuel consumption and emissions over the equivalent petrol or diesel models,” the company told Autocar in a statement.

JLR, meanwhile, pointed to the chunky batteries in Range Rover and Range Rover Sport plug-in hybrids. “Typical Range Rover customers will be able to complete up to 75% of trips using electric power only if they begin each journey with a full charge,” the company said in a statement to Autocar. “Plug-in hybrids offer clients a great combination of performance, fuel economy, low emissions, with zero emissions at the tailpipe.”

Automotive supplier group CLEPA took issue with the results. “The [European] Commission’s recent study does not account for technological progress,” the organisation said in an email to Autocar. It defended the technology as an important addition to the choices available to consumers as they make their shift to lower-CO2 technologies. “Plug-in hybrid electric vehicles should be seen as a bridge towards electrification, not a U-turn,” it said.

We also approached key plug-in hybrid car makers Mercedes-Benz, Volvo and Ford for comment but haven’t received a response. No car maker we approached responded to our request for internal data showing average CO2 figures for their fleet.

The PHEV hasn’t been mortally wounded by this report – far from it. As CLEPA points out, the data is relatively out of date, given it was harvested from cars first registered in 2021. Many car makers didn’t report data, meaning the numbers were dominated by just a handful of brands. The plug-in hybrid dataset consisted primarily of Mercedes-Benz (39%), Volvo (19%), and Ford (16%) vehicles. The European Commission admitted the flaws. “At this point, it is too early to identify trends in the gap size as data is only available for one year,” the commission said.

The purpose of the data collection is to work out whether authorities in the European Union (and the UK, given it also uses WLTP figures to calculate CO2) need to change the way they calculate CO2 figures. 

That’s already going to happen. The “mismatch” between the so-called utility factor – that is, the percentage of time the test assumed the car will be driven in electric mode – and the actual drive time is going to be narrowed from 2025. Then, the test will assume the car will be driven half its time on electric, rather than the current 70-85% assumption. A further change is expected in 2027, informed by another interrogation of figures this year.

That’s going to remove some of the more fantastical CO2 and MPG claims from new plug-in hybrids sold. For example, reducing the expected electric miles driven in the changed 2025 test (the so-called Euro 6e-bis) will increase the recorded CO2 for the BMW X1 xDrive25e, a plug-in hybrid SUV, from 40g/km to 96g/km, the International Council of Clean Transportation (ICCT) calculated. The proposed change in 2027 would increase that again to 122g/km.

With those numbers, the PHEV would wither and die. The cost to integrate an expensive battery and electric motor into a combustion engine car would blow the business case out of the water without the attendant low CO2 figure to reduce average emissions and offer some tax benefits to the customer

However, car makers are responding by increasing the battery size to give a greater electric-only range, meaning that they’ll be able to match the pace of the WLTP changes to the electric ‘utility factor’.

Battery sizes grew by a “massive” 27% between August 2022 and the same month last year, according to the most recent study by metals and minerals consultancy Adamas Intelligence, bringing the average battery size to 21.8kWh.

Examples include Mercedes’ recent overhaul of the GLC to increase the battery size in the 300e PHEV to 24.8kWh from 13.5 kWh, boosting the claimed electric-only range to 81 miles from 28 miles. The pack – which is slightly larger than that fitted to the Mk1 generation Nissan Leaf EV – has also gone into the firm's new E300e and C300e.

Meanwhile, the Volkswagen Group has increased the size of the battery in its new plug-in hybrid cars on its MQB Evo platform from 13kWh to 19.7kWh, including the VW Tiguan, VW Passat, VW Golf, Skoda Kodiaq and Skoda Superb. In the Golf, that gives a claimed electric range of 62 miles.

The effect, however, has also been to make plug-in hybrids even more expensive, given the need to package an even bigger battery along with an electric motor and regular combustion engine. It’s probably no coincidence that three best-selling plug-in hybrid brands – Land Rover, Audi and BMW – in the first three months of UK sales were all premium. The Audi A3 was the UK’s top-selling PHEV in the first three months.

The technology has even pulled in luxury brands including Bentley and Aston Martin, both of whom have pushed back plans to launch electric cars in favour of an expanded plug-in hybrid range. The technology suits customers who "want some electrification but still have the sports car smell and feel and noise,” Aston Martin chairman Lawrence Stroll told investors recently.

The poor real-world figures versus the test results are partially linked to their strong fleet usage. In the first half of this year, three-quarters of all plug-in hybrids were bought by companies. Incentivised by the low CO2 figure extrapolated from the test, company car drivers often lack the follow-on incentive to actually charge the battery and achieve the quoted figure, or even get close to it.

In fact, company cars are driving on electric just 11-15% of the time, according to the green transport pressure group T&E, meaning there’s still a way to go before the real figure is reached. “This is particularly problematic as company cars make up 71% of new PHEV sales,” T&E said.

If PHEVs were only a European phenomenon, they might have died on the development table, squeezed on one side by rising sales of hybrids and on other by EVs as they become cheaper. In the UK, despite the sale growth, they still accounted for just a 7.8% market share in the first three months, compared with 15% for EVs and 14% for hybrids.

However, their growing popularity in China (half of all BYD sales there are PHEVs) and the US are likely to mean they will continue in Europe as well. One possible path before the door  shuts for good in 2035 is the range-extender, where the battery and electric motor are the dominant motive force, aided by a small petrol engine to fill in charging gaps. Use of the technology has empowered China’s Li Auto, a premium start-up that actually makes money. It’s also found in the LEVC London taxi. 

Meanwhile, a fairer assessment of their real-world contribution to CO2 emissions could even save the PHEV if it convinces authorities they should remain a tax-friendly option.