Plug-in hybrids that are rarely charged: why T&E wants a 15-litre fuel tank cap

What the report found

Transport & Environment (T&E), the Brussels-based clean transport campaign group, published "Still smoking: the PHEV emissions scandal continues" on 15 October 2026. It analyses real-world fuel data that the European Environment Agency (EEA) collects from plug-in hybrids registered in 2024. On average, their real-world CO2 emissions were nearly six times their official WLTP values, up from roughly five times for 2023 registrations.^1,2,9,11

T&E's September analysis of the same EEA release put the 2024 averages at 145 gCO2/km on the road against 24 g/km on paper, based on fuel monitors in about 220,000 plug-in hybrids. Real-world emissions rose by about 5% from 2023 while official figures fell by about 15%. Conventional combustion cars averaged 169 g/km, so plug-in hybrids came out only 14% lower in practice, against an 83% gap in the official numbers.^1,3,8

The trend is not new. The European Commission's first report on this data, published in March 2024, found that plug-in hybrids first registered in 2021 emitted on average 3.5 times their type-approval CO2. It attributed the gap mainly to cars not being charged and driven electrically as often as the official test assumes.⁴

Where the numbers come from

Since January 2021 every new car and small van in the EU that can run on liquid fuel must carry an on-board fuel consumption monitor (OBFCM). The device records lifetime fuel use and distance. For plug-in hybrids it also logs distance in charge-depleting mode with the engine on and off, fuel used in that mode and the grid energy put into the battery.^4,5

Manufacturers read the meters over the air or at dealer visits, and from 2023 member states also read them at periodic technical inspections. The data go to the EEA, which links them to each car's laboratory figures and publishes anonymised raw and aggregated files a year later. EU law asks the Commission to propose a method for using this data to adjust carmakers' CO2 results from 2030, with a report due by 31 December 2026.⁵

The figures carry caveats. T&E is a campaign group, and its own annex notes that some brands can be over- or under-represented in a given year's sample, naming BMW in 2024. Weighting by registrations narrows the 2024 gap from 6.1 times to 5.3 times, but the upward trend remains.¹

Why the gap is widening

The official CO2 figure for a plug-in hybrid depends on the utility factor, an assumed share of driving done on electricity that rises with tested electric range. T&E's reading of the 2024 data is that cars covered 27% of their distance in pure electric mode, and that grid electricity supplied 30% of their energy. The utility factor in force for 2025 and 2026 assumes 56%.¹

Even when the battery was supplying power, the engine ran for about a third of charge-depleting kilometres. Real-world emissions in that mode averaged 63 gCO2/km, about 2.8 L/100 km, against 6 g/km in the official figure.^1,2

Longer electric range has not closed the gap on its own. Average tested range rose from 58 km in 2021 to 75 km in 2024, and the share of new plug-in hybrids above 70 km rose from 13% to 49%. Those longer-range cars were heavier and had more powerful engines, emitted about 200 g/km once the battery was empty, and accounted for more than half of the 2024 gap.¹

The stronger signal was motor power. Plug-in hybrids whose electric motor had more than 1.25 times the engine's power averaged 108 gCO2/km on the road, compared with 227 g/km for those below 0.5. The average ratio was 0.7 and has barely moved since 2021.¹

The 15-litre tank proposal

T&E wants tax rules and type approval to favour what it calls well-designed plug-in hybrids. Its criteria are an electric motor with at least three times the engine's power, DC charging above 100 kW, at least 200 km of real-world electric range by 2030 with the car defaulting to electric drive, and a fuel tank of 15 litres or less. Its 2025 report put the average plug-in hybrid tank at about 51 litres.^1,6

The logic is that a small tank gives drivers a reason to plug in. T&E calculates that 15 litres at 6.7 L/100 km, the average it found for range-extender cars once the battery is depleted, would give about 220 km of engine-only driving, roughly matching the electric range it wants.^1,6

The proposal sits alongside a fight over the utility factor itself. The EU's second correction is due for new type approvals on 29 November 2026 and all new cars on 29 November 2027. For a car with 100 km of tested range it lowers the assumed electric share from about two-thirds to 44%. T&E says freezing it at the current level would understate real-world emissions by 56%.^1,2,7

What a smaller tank would mean in practice

Charjin worked the numbers using only T&E's inputs: 6.7 L/100 km in engine mode, 12,730 km a year and EU petrol at €1.86 a litre, the March to October 2026 average. A 51-litre tank covers about 760 km between fills and costs about €95 to fill. A 15-litre tank covers about 224 km and costs about €28.^1,6

For a driver who never charges, that is the difference between about 17 and about 57 stops a year at the pump, for the same fuel bill of roughly €1,590. The small tank changes the hassle of not charging, not the cost of each kilometre. For a driver who charges every night and makes a few long trips, the extra stops would only come on those trips.

T&E's cost estimate points the same way. At 2026 fuel prices it calculates that the average 2024 plug-in hybrid costs €1,810 a year to run, against €1,130 implied by the official figures used at the time of sale. That gap of €680 falls to €515 under the utility factor in force since 2025.^1,2

How to judge a plug-in hybrid now

The best predictor is your own charging. If you can charge at home or work and most of your days fit within the real electric range, a plug-in hybrid can do much of its work on electricity. If you cannot, a conventional hybrid or an electric car may suit you better than the official CO2 figure suggests.¹

Check the electric motor's power against the engine's, not only the range. T&E names the BYD Seal U DM-i as the 2024 model with the highest ratio, 1.64, and a 20% gap against the corrected WLTP basis. BYD's European page still advertises up to 1,080 km of combined range alongside a 70 km-plus electric range, so even the best-scoring car would sit far from a 15-litre design.^1,10

Company car drivers should also note that tax perks often follow the official number. Companies registered 71% of new EU plug-in hybrids in the first half of 2026, and 24 of 27 member states give them a tax advantage, but only 12 set a threshold of 50 gCO2/km or lower. Whether any government adopts a tank limit is unknown; none has announced one.¹