Study: Plug-in hybrids emit six times more CO₂ in real-world use than officially stated
The gap between official and actual CO₂ emissions of plug-in hybrids is widening further. According to a new analysis by Transport & Environment, PHEVs registered in 2024 emitted on average about six times as much CO₂ in real-world use as reported officially.
According to T&E, actual PHEV emissions are far higher than the official figures.
Image: Florian Treiß
Transport & Environment (T&E) based its analysis on data from the European Environment Agency. According to this, the actual CO₂ emissions of PHEVs registered in 2024 were on average about six times higher than the official values. In 2023, the factor was around five according to T&E.
The discrepancy also has financial implications for drivers. T&E estimates that the additional fuel costs compared to the official fuel consumption figures for a 2024 PHEV amount to an average of 680 euros per year. Even when compared to conventional internal combustion engines and non-external chargeable hybrids, the real-world advantage is smaller than what the official figures suggest. According to T&E’s analysis, the actual emissions of plug-in hybrids were only 14 percent lower than those of these other vehicles. The official test figures, however, suggested a reduction of 83 percent.
According to T&E, one reason for this is that the internal combustion engine keeps running regularly even during what appears to be electric operation. The analysis showed this occurred in about one third of the kilometers driven in battery mode.
The fact that the discrepancy between test conditions and real-world use is not a new problem was already demonstrated by the International Council on Clean Transportation (ICCT) last summer. In a study of around eight million vehicles, researchers found that the actual emissions of PHEVs in 2023 were on average about five times higher than the official test values. The ICCT cited factors such as lower electric driving range than assumed, frequent use of the internal combustion engine, and the additional weight from the dual-drive system as causes.
Dispute over the factor for CO2 reduction
The core of regulation is the so-called efficiency factor. It determines what proportion of a plug-in hybrid’s driving range is considered electric when calculating official CO2 emissions. The EU has already adjusted this factor to bring the type test values closer to actual usage.
The automotive industry is now also exploring how to technically ensure a higher electric driving proportion. The VDA proposed in 2025 that future plug-in hybrids be designed to require regular charging. If charging does not occur within a yet-to-be-determined driving distance, the power output could be reduced. This proposal aims primarily to address the argument that the actual electric driving proportion of PHEVs has so far been difficult to guarantee.
T&E already identifies a technical effect in the first correction. The organization analyzed 20 of the best-selling PHEV models, which together accounted for 51 percent of sales in 2025. According to this analysis, manufacturers responded by using larger batteries and increasing electric range, among other measures.
According to T&E, the adjustments were particularly evident in Volkswagen Group models. In the VW Tiguan, the electric range increased from 55 to 119 kilometers. This allowed the official emissions to remain at 36 g CO2/km despite stricter calculation methods, down from previously 32 g/km. In five tested VW models, around 90 percent of the increase in official emissions caused by the duty factor adjustment was offset this way.
T&E believes manufacturers could also address the next round of adjustments through further technical improvements. These might include greater electric range, more efficient combustion engines, reduced weight, better aerodynamics, and more effective regenerative braking. The organization thinks it is possible to keep PHEVs below the 50 g CO2/km threshold in this way.
Politically, however, the next correction is controversial. T&E warns against freezing the fuel efficiency figure at the 2025/26 level. According to the organization’s calculations, real PHEV emissions would still be underestimated by 56 percent under such circumstances.
This would have implications not only for the fuel consumption figures presented to customers. Lowly estimated PHEV emissions also affect manufacturers’ CO2 fleet balances, thereby influencing how many pure electric vehicles need to be sold to meet European fleet targets.
The ICCT had also previously pointed out that the current corrections are insufficient to fully reflect the actual emissions of PHEVs. Thus, both analyses arrive at a similar conclusion despite using different data sets. This is because what matters most for the climate impact of plug-in hybrids is not only their technical design but, above all, how frequently they are actually driven electrically and charged.