Scientists from the Kore University of Enna (Enna, Italy) and the Rzeszow University of Technology (Rzeszow, Poland) have completed Europe’s most ambitious study of the real-world performance of plug-in hybrid electric vehicles (PHEVs). They analyzed on-board monitoring data from 457,303 vehicles for 2021–2023 and compared actual engine fuel consumption and CO₂ emissions with the official type-approval values. This study allowed scientists to assess, for the first time on such a large scale, in what way the use of PHEVs on the roads differs from test conditions, and draw conclusions with direct implications for the EU’s climate policy.
The researchers revealed a profound discrepancy between official test values and actual use. According to certification calculations, average CO₂ emissions per PHEV stand around 46 g/km. However, in real-world use, this figure reaches approximately 138 g/km, or almost three times as high. By comparison, this gap is larger than the one observed in conventional gasoline vehicles (15-20%). The primary reason lies in the faulty assumptions of the test cycle, i.e. regulatory expectations of 80% battery usage during daily driving. The actual data reveal a different picture: on average, PHEVs operate on battery power for 45.5% of their total driving distance, with the remaining 54.5% powered by the combustion engine. This is related to both limited availability of charging infrastructure and peculiarities of the drivers’ behavior, which range from the fear of draining the battery during a long journey to a simple preference for driving on gasoline. Moreover, increasing battery capacity does not guarantee improved performance: heavier batteries often lead to even greater discrepancies between test results and reality.
To obtain their findings, the scientists leveraged the European Union’s On-Board Fuel Consumption Monitoring (OBFCM) database. To weed out errors and anomalies, they applied rigorous statistical methods, and conducted a multi-dimensional analysis, comparing the performance across different brands, tracking trends over three years, and building regression models. It turned out that the mean regulatory gap between the test and reality is not only significant, but exhibits a troubling upward trend, worsening every year, from 258% in 2021 to 368% in 2023. Meanwhile, the absolute values of real emissions remain virtually unchanged, indicating a disturbing tendency: apparently, automakers have learned how to “trick” lab tests by optimizing software without, however, improving the actual environmental performance of their vehicles.
The disparity in the results between brands was particularly telling. This proves that, using technological advances, it is possible to achieve much more accurate results, but not all manufacturers are aiming for these improvements. Even Scandinavian and German premium brands, traditionally associated with high quality and innovation, show subpar results. Thus, a car’s prestige and price do not guarantee its real-world environmental friendliness.
At the end of the day, the scientists concluded that the current type-approval procedures are not only outdated but also might have adverse climate policy impacts. The existing system creates a false sense of progress, with hybrids only nominally meeting “green” standards.
To address this problem, the scientists propose introducing mandatory Real Driving Emissions (RDE) testing for PHEVs, similar to NOx compliance requirements recently imposed on diesel vehicles, and revision of utility factor assumptions from the current fantastical 84% down to 34%, to reflect realistic driver battery charging behavior. According to the calculations of the authors of the study, closing this gap could deliver up to 61% of the EU’s required transport sector emissions reductions by 2030, yielding annual EU-wide CO2 savings of 555 million tons.
