Real-World PHEVs Guzzle Six Times More CO2 Than Lab Tests Claim, Analysis Finds

Plug-in hybrids have long been marketed as a practical bridge between combustion and full electrification, but a fresh analysis suggests that bridge is far shakier than official numbers imply. Transport & Environment (T&E), drawing on data from the European Environment Agency, reports that PHEVs registered in 2024 pumped out roughly six times more CO2 on real roads than type-approval figures indicate. That gap has widened: a year earlier, T&E put the multiplier at about five.

The financial sting lands squarely on owners. T&E estimates that a PHEV bought in 2024 burns through an extra €680 in fuel annually compared with what official consumption ratings would lead drivers to expect. Worse, the environmental edge over ordinary combustion cars and non-plug-in hybrids is thinner than advertised. Measured against those vehicles, real-world PHEV emissions came in only 14 per cent lower — while the official test cycle claimed an 83 per cent reduction.

Why the divergence? Much of it comes down to how often the engine fires up when the car is supposedly running on battery power. T&E found the combustion unit was active for roughly one-third of the kilometres driven in electric mode. This is not an isolated finding. The International Council on Clean Transportation (ICCT) reached similar conclusions this summer after studying some eight million vehicles, attributing the shortfall to weaker-than-assumed electric performance, frequent engine use, and the extra mass carried by a dual powertrain.

At the regulatory heart of the issue sits the ‘utility factor’ — the assumed share of electric driving baked into official CO2 calculations. Brussels has already tightened this factor, and automakers have responded. T&E examined 20 top-selling PHEV models, which together represented 51 per cent of 2025 sales, and found manufacturers countering with bigger batteries and longer electric ranges. The shift was starkest at the Volkswagen Group: the VW Tiguan’s electric range jumped from 55 to 119 km, letting it hold official emissions at 36 g CO2/km under the stricter method, versus 32 g/km before. Across five VW models studied, roughly 90 per cent of the increase triggered by the utility factor correction was offset this way.

T&E argues further tightening is technically feasible — through longer electric range, leaner combustion engines, weight reduction, better aerodynamics and smarter recuperation — and that PHEVs could stay under the 50 g CO2/km mark. But the next correction is politically charged. T&E warns against freezing the utility factor at its 2025/26 level, calculating that real-world PHEV emissions would still be underestimated by 56 per cent. That matters beyond sticker figures: inflated electric assumptions feed into manufacturers’ fleet CO2 balances, shaping how many pure EVs they must sell to hit European targets. Ultimately, the climate case for plug-in hybrids rests less on engineering than on whether drivers actually plug them in.