Plug-in hybrid vehicles (PHEVs) registered in Europe in 2024 emitted an average of around six times more CO₂ under real-world driving conditions than official figures indicated, according to a new analysis by environmental organisation Transport & Environment (T&E).
The analysis uses data from the European Environment Agency (EEA) and highlights a widening gap between laboratory test results and emissions recorded during everyday driving. T&E reported that the difference was approximately fivefold for vehicles registered in 2023.
The discrepancy also affects running costs. According to T&E, drivers of PHEVs sold in 2024 incurred an average of €680 in additional annual fuel costs compared with what official consumption figures would suggest.
The organisation estimates that real-world PHEV emissions were only 14% lower than those of conventional combustion-engine vehicles and non-plug-in hybrids. Official test figures, by comparison, suggested an 83% reduction.
Combustion Engines Used During Electric Driving
T&E identifies the continued use of combustion engines during journeys recorded as electric driving as one reason for the difference. Its analysis found that the internal combustion engine was running during approximately one-third of the distance driven in battery mode.
The findings are consistent with a separate analysis by the International Council on Clean Transportation (ICCT), which examined data from around eight million vehicles. The ICCT found that real-world PHEV emissions in 2023 were, on average, approximately five times higher than official test values.
Factors cited by the ICCT included lower-than-expected electric driving shares, frequent use of combustion engines and the additional weight associated with combining electric and combustion powertrains.
EU Utility Factor Under Scrutiny
A central element of the EU’s method for calculating PHEV emissions is the utility factor. This determines the proportion of a vehicle’s total mileage assumed to be covered by electric driving when official CO₂ figures are calculated.
The EU has already adjusted the utility factor to bring laboratory results closer to real-world use. T&E says this change has prompted manufacturers to make technical improvements, particularly by increasing battery capacity and electric driving range.
The organisation examined 20 of the best-selling PHEV models, which together accounted for 51% of sales in 2025. Its analysis found that manufacturers had responded to the revised calculations with changes intended to increase electric driving capability and limit the impact on official emissions figures.
Volkswagen Models Illustrate the Effect
T&E identified particularly significant changes in models from the Volkswagen Group. The electric range of the Volkswagen Tiguan plug-in hybrid increased from 55 km to 119 km, according to the analysis.
Despite stricter utility-factor calculations, the model’s official CO₂ emissions were reported at 36 g/km, compared with 32 g/km previously.
For five Volkswagen Group models examined by T&E, technical improvements reportedly offset around 90% of the increase in official emissions resulting from the revised utility factor.
T&E argues that further engineering changes could help manufacturers respond to another adjustment. Potential measures include increasing battery capacity and electric range, improving combustion-engine efficiency, reducing vehicle weight, optimising aerodynamics and enhancing regenerative braking.
The organisation believes these measures could allow manufacturers to keep some PHEVs below the 50 g CO₂/km threshold. However, the extent to which such improvements translate into lower real-world emissions will also depend on vehicle use and charging behaviour.
Debate Over Further Regulatory Changes
T&E has warned against freezing the utility factor at the 2025–2026 level. According to its calculations, doing so would leave real-world PHEV emissions underestimated by 56%.
The issue has implications beyond the figures presented to consumers. Official PHEV emissions estimates contribute to manufacturers’ fleet-wide CO₂ calculations, influencing how automakers demonstrate compliance with European emissions targets and plan their sales mix.
In 2025, the German automotive industry association VDA proposed that future plug-in hybrids should be designed to require regular charging. Under the proposal, a vehicle’s powertrain output could be reduced if it had not been charged within a distance yet to be defined. The idea was intended to encourage electric driving, although it would require further technical and regulatory development.
The ICCT has also argued that the latest adjustment to the utility factor does not fully reflect real-world emissions.
Together, the analyses point to a persistent challenge in measuring the climate benefits of plug-in hybrids. Their actual emissions depend not only on battery capacity and powertrain design, but also on how often drivers charge the vehicles and how much of their mileage is completed using electric power.