Real-world CO2 emissions from plug-in hybrids are now six times higher than WLTP figures

A fresh analysis by Transport & Environment (T&E), based on European Environment Agency data, covers around 220,000 plug-in hybrids registered in the European Union in 2024. The vehicles’ onboard fuel- and energy-consumption monitoring systems show just how far laboratory testing has now drifted from real-world use.

However, the most important figure is not the sixfold difference, but 145 g/km itself. The real-world average for conventional internal-combustion-engine cars was 169 g/km. Plug-in hybrids therefore emit only around 14% less CO2 in traffic than a car without a charging port or large traction battery.

That makes the environmental advantage of a plug-in hybrid much smaller than the WLTP figures in the price list suggest.

Better on paper, worse on the road

The trend is even more troubling. Plug-in hybrids registered in 2023 had an official average CO2 figure of 28 g/km, while emitting around 138 g/km in real-world driving. A year later, the official figure improved to 24 g/km, but real-world emissions instead rose to 145 g/km.

WLTP emissions therefore fell by around 14% over the year, while actual emissions increased by roughly 5%.

This is no longer a random measurement error. The European Union has collected real-world fuel-consumption data since 2021, and the gap between official and real-world CO2 emissions has consistently widened.

The ICCT reached the same conclusion in an even larger analysis published in September. The organisation examined onboard-computer data from around 920,000 plug-in hybrids registered in Europe between 2021 and 2023. For 2021 cars, real-world CO2 emissions exceeded the official figure by around 260%; for 2023 cars, they were already more than 400% higher.

Longer electric range has not solved the problem.

WLTP assumes electric driving that does not happen in real life

The core of the issue is not necessarily plug-in-hybrid technology, but the way Europe calculates its CO2 emissions.

A so-called utility factor is used to determine a plug-in hybrid’s official figure. In essence, it predicts what share of the distance driven is covered with a charged battery and predominantly electric power.

The longer a vehicle’s electric range, the greater the share of electric driving assumed by the calculation. This, in turn, drives the official fuel-consumption and CO2 figures very low.

Real users behave differently.

ICCT data show that while the electric range of plug-in hybrids has increased over the years, the actual share of driving completed electrically has not risen as expected. On the contrary, it has fallen.

The reason is hardly mysterious. A plug-in hybrid performs at its best only when its owner charges it regularly and daily trips largely fit within its electric range. If the charging cable stays in the boot, the vehicle carries an internal-combustion engine, gearbox, fuel tank and heavy battery. In that case, the technically complex economy car simply becomes a heavy petrol car.

Six times does not mean six times more than a petrol car

The headline figure in T&E’s findings needs clarification here.

At 145 g/km, the figure is roughly six times higher than plug-in hybrids’ official 24 g/km rating. It does not mean that a plug-in hybrid pollutes six times more than a conventional internal-combustion-engine car.

On the contrary, T&E’s data show that petrol- and diesel-powered cars emitted an average of 169 g/km. Plug-in hybrids’ 145 g/km still gives them an advantage, but it is only 14%.

In the 2023 data, the difference was still around 17%. Plug-in hybrids are therefore not only further from their official figures, but their real-world CO2 advantage over conventional cars has also narrowed.

At the same time, it would be wrong to declare the entire technology pointless. In a well-chosen usage profile, a plug-in hybrid can work very efficiently. An owner who charges the vehicle every night, drives, for example, 30-60 km on weekdays and uses the internal-combustion engine mainly on long trips is using the powertrain much as its engineers intended.

The problem arises when that same car goes into a company fleet, there is no charger at home, and a vehicle with 100 km of electric range sees its charging cable once a week.

WLTP cannot distinguish between these two owners.

Even EV mode does not always mean the petrol engine is inactive

There is another uncomfortable detail in plug-in hybrids’ real-world emissions. An earlier T&E analysis of 127,000 plug-in hybrids found that, even during driving classified as electric, the cars used an average of around 3 l/100 km of fuel.

As a result, average CO2 emissions in electric driving mode reached 68 g/km, around 8.5 times above the official assumption. The internal-combustion engine supplied energy for almost a third of the distance covered in EV mode.

This can happen due to high power demand, high speed, a steep climb, low temperatures or cabin heating. Naturally, the control logic of all plug-in hybrids is not identical, but the general problem remains: an EV-mode indicator on the dashboard does not always guarantee that nothing is coming out of the exhaust pipe.

Europe wants to correct the calculation error

These figures are particularly important because of European Union CO2 regulations. A plug-in hybrid’s low official emissions affect not only the owner’s specification sheet, but also help a carmaker reduce the calculated average CO2 emissions of its entire fleet sold.

The European Commission has acknowledged the problem and began adjusting the utility factor in 2025. The next significant change is expected in 2027, to reduce the gap between the WLTP assumption and the vehicles’ real-world use.

For the automotive industry, this is far from a minor accounting adjustment. If the same plug-in hybrid’s official CO2 figure rises sharply because of a more realistic calculation method, its value as a means of reducing a manufacturer’s fleet emissions will diminish.

According to T&E, the European automobile manufacturers’ association ACEA is pressuring the European Union to cancel the planned 2027 change. At the same time, plug-in hybrids already accounted for 9.8% of EU new-car sales in the first half of 2026.

This is where a technical measurement problem quickly becomes industrial policy worth billions of euros.

A plug-in hybrid is not a bad car, but the wrong owner can make it one

Plug-in hybrids are not disappearing from the European market. On the contrary, increasingly stringent fleet CO2 requirements and a slower transition to electric cars are making them an attractive interim solution for manufacturers.

The technology itself is also advancing. Newer models already offer more than 100 km of electric WLTP range, more powerful electric motors and, in some cases, fast DC charging. Used correctly, such a car can cover most daily driving without starting its petrol engine.

But Europe’s real-world usage data show that technical capability and owner behaviour are two different things.

That is the fundamental weakness of the plug-in hybrid. An electric car forces its owner to use electricity. A petrol car forces them to refuel. A plug-in hybrid offers a choice, and European data show that too many users choose a method in which all the complex twin-powertrain technology delivers a surprisingly small CO2 advantage.

That is why 145 g/km is less an indictment of plug-in-hybrid engineers than a blow to the WLTP calculation model. When the official figure says 24 g/km and 220,000 real cars average 145 g/km, the problem is no longer a statistical anomaly.

The measurement method simply describes a car that does not actually exist on European roads.