New research shows that real‑world CO2 output from plug‑in hybrids can be up to 4.6 times higher than the figures reported in the official WLTP test cycle.
Official tests versus everyday driving
The International Council on Clean Transportation (ICCT) found a gap of 3.5 to 4.6 times between certified values and actual performance. The WLTP cycle, used across Europe to gauge fuel use, appears to underestimate the pollution from these vehicles.
Such a disparity raises doubts about the reliability of certification data as a measure of environmental impact for everyday use.
Data collected from nearly a million cars
The analysis draws on onboard fuel and energy monitoring (OBFCM) devices installed on almost 920,000 new cars registered between 2021 and 2023. These “black boxes” record fuel and electricity consumption continuously and cannot be altered.
Researchers started with roughly 1.8 million raw records, removed anomalies, and then re‑weighted the figures according to each model’s market share, preventing distortion from any single popular type.
Why the gap exists
Drivers tend to charge far less often than the test assumes, so the proportion of miles run on electric power is markedly lower. When the battery runs low, its weight plus the electric system’s components add extra load, pushing fuel use higher.
The study notes that this weight effect is a physical factor that compounds the higher fuel burn caused by reduced electric driving.
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Corporate fleets feel the sting
Company‑owned vehicles show an even larger gap because daily charging rates are historically low. Reimbursement policies that favor fuel over electricity further discourage the use of electric mode, leading to greater reliance on the combustion engine.
According to the ICCT, these practices amplify the disparity between test results and real‑world output for fleet operators.
Looking ahead, regulators may need to tighten incentives for regular charging if they want the official numbers to reflect reality. Aligning driver behavior with test assumptions could narrow the gap, but it will likely require changes to both policy and corporate reimbursement structures.
European response
The European Commission plans methodological adjustments to the utility factor (UF) calculations starting in 2025, with further refinements slated for 2027. These changes aim to bring official figures closer to the data gathered by OBFCM units.
By incorporating real‑world monitoring into the certification process, officials hope to provide a more accurate picture of CO2 release from plug‑in hybrids.
Manufacturers now face growing pressure to ensure that advertised low‑emission claims match actual performance on the road.
