Plug-in hybrids occupy an awkward position in emissions policy. They can be extremely clean or barely better than a conventional car, and the difference is decided by owner behaviour rather than by engineering.
What the design assumes
A plug-in hybrid carries a battery large enough for typical daily driving on electricity alone, with an engine available for longer journeys.
The design assumes the owner plugs in regularly, so most short trips are electric and the engine handles only occasional distance.
Under that pattern the car genuinely delivers very low fuel consumption, which is what the concept was built to achieve.
Why the official figures overstate it
Test procedures measure a car over a defined cycle starting with a full battery, then apply a weighting to account for how often it is expected to run on electricity.
That weighting is an assumption about behaviour, and the resulting figure is extremely low because much of the test distance is covered electrically.
A car that is never charged uses the engine for everything while carrying the additional weight of a battery and motor, and its actual consumption bears little relation to the published number.
Where the incentive problem arises
Company car taxation, purchase incentives and manufacturer fleet targets in several markets are calculated from the official figure.
That creates a situation where a driver can obtain a substantial tax advantage without ever using the electric capability, since the benefit attaches to the car rather than to how it is used.
Fleet data has repeatedly shown a wide gap between certified and observed consumption, which is what prompted regulators to revisit the rules.
How the rules are being tightened
The weighting factors used in certification have been revised to assume less electric running, which raises the official figures for these cars considerably.
Minimum electric range requirements have also been raised, on the reasoning that a longer range makes electric operation practical for more journeys.
Onboard fuel consumption monitoring now allows real-world data to be gathered across fleets and compared against certified values.
Why the technology still has a role
For a driver whose regular journeys fall within electric range and who has reliable charging, a plug-in hybrid removes most fuel use without any concern about longer trips.
They also demand far less battery material per car than a full electric vehicle, which matters where cell supply is the constraint.
The dispute is not about whether the layout can work. It is about whether policy should reward a capability that may never be used.