Plug-in hybrid electric vehicles (PHEVs) were marketed as an efficient bridge between traditional combustion engines and full electric cars. However, a recent study reveals that drivers are using roughly three times more fuel than manufacturers and regulators claim. The discrepancy highlights a critical gap between laboratory testing and real-world driving conditions.
The Disconnect Between Lab and Road
Researchers at the Fraunhofer Institute analyzed data from over 981,000 PHEVs produced between 2021 and 2023. This is the largest real-world study to date, using wireless vehicle data to compare official WLTP fuel consumption figures with actual driving patterns. The results are stark: PHEVs average 6.12 liters per 100 kilometers (38.4 mpg US) in practice, compared to a certified 1.57 L/100 km (149.8 mpg US).
Even when operating in “charge-depleting” mode—when the car should rely primarily on electric power—fuel consumption averaged 2.98 L/100 km (78.9 mpg US), almost double the official combined figure.
Brand Performance Varies Widely
The study also exposed significant variations between manufacturers. High-end German models, particularly Porsche plug-in hybrids, recorded some of the highest real-world fuel consumption, averaging around 7 L/100 km (33.6 mpg US). In contrast, lower-priced PHEVs from brands like Kia, Toyota, Ford, and Renault often achieved fuel economy under 1 L/100 km (over 235.2 mpg US), roughly 85% lower than Porsche.
Porsche defends its figures by stating that they comply with EU testing procedures, attributing the differences to individual usage and driving conditions. The European Commission and Germany’s automotive industry association have yet to comment on the study’s findings.
Why the Gap Exists
The core issue? Drivers don’t charge their PHEVs as frequently as regulators assume. Real-world data indicates that private PHEVs are driven electrically about 45–49% of the time, while company cars drop to just 11–15%. WLTP calculations rely on an electric driving share of 70–85%. Without regular charging, the combustion engine must compensate, carrying the added weight of an uncharged battery.
Additional factors include shorter real-world electric range due to cold weather, high speeds, and elevation changes, as well as drivers exceeding the vehicle’s advertised electric range. The combustion engines themselves may also burn more fuel in real-world conditions than in lab tests.
The Broader Implications
This study confirms long-standing doubts about the accuracy of laboratory fuel economy figures. It raises questions about whether PHEVs are delivering the environmental benefits they were intended to provide, particularly if drivers fail to fully utilize the electric component.
The issue isn’t limited to PHEVs alone. As Patrick Plötz of the Fraunhofer Institute notes, “the engine appears to engage far more often than manufacturers implied.” This underscores the importance of real-world testing and the need for more realistic regulatory standards.
Ultimately, a plug-in hybrid only delivers on its promise if the driver plugs it in. Without consistent charging, the PHEV becomes a heavier, less efficient version of a traditional hybrid.
