The PHEV utility factor, explained — and why longer electric range has not fixed the gap

A plug-in hybrid's official fuel figure is not a measurement of how the car is driven. It is a blend of two lab results — one with a charged battery and one with an empty battery — weighted by an assumed share of electric driving. That assumed share is the utility factor.

How the utility factor works

The longer a car's official electric range, the higher its utility factor, and the lower its official fuel figure. For a car with about 60 km of range, the test assumes more than 80% of driving is electric. In the EEA's 2021–2023 fuel meter data, plug-in hybrids actually covered about a quarter of their distance on battery alone.

What the 2024 data shows

We grouped all 160 plug-in hybrid models with reliable data by their official electric range. Longer range brings the official figure down sharply. The share of distance owners actually drive with the engine off barely moves.

In other words, bigger batteries have mostly improved the paperwork. The official figure falls, the real figure does not, and the gap between them grows.

How the rules are changing

The EU is correcting the utility factor in two steps. The first came with the Euro 6e-bis standard in 2025, which carmakers answered largely by fitting bigger batteries. The second correction is due from November 2026, although Transport & Environment reports that Germany has pushed to freeze it.

In the UK, HMRC applies a temporary benefit-in-kind easement so that qualifying Euro 6e-bis plug-in hybrids are not pushed into a higher company car tax band by the new test. See our company car tax guide.

What it means for you

The official figure tells you what the car can do if it is charged before almost every trip. The real-world figure on each model page tells you what owners actually get.