Hybrid consumption figures in town are often better than on the motorway, which reverses the pattern for a conventional car. The reason lies in where a hybrid's savings actually come from.
Braking energy is the main recovery
Slowing a car converts its kinetic energy into heat at the brakes, and in a conventional vehicle that energy is entirely lost to the air.
A hybrid instead uses its electric machine as a generator, capturing a substantial portion of that energy and returning it to the battery for reuse.
Urban driving involves constant acceleration and braking, so the opportunity to recover arises repeatedly, and each recovery reduces the fuel needed for the next acceleration.
Idling is eliminated rather than reduced
A combustion engine consumes fuel while stationary and produces nothing useful. In congested traffic that can account for a considerable share of a journey's fuel.
A hybrid shuts the engine down whenever it is not needed, and because the electric system supplies climate control and accessories, the shutdown can persist.
Restarting is handled by the main electric machine rather than a conventional starter, so it is quick and smooth enough for the driver to ignore.
The engine is kept in its efficient region
A combustion engine is most efficient in a fairly narrow band of speed and load, and normal driving spends much of its time well outside that band.
With an electric machine available to add or absorb torque, the control system can hold the engine closer to its efficient region and buffer the difference through the battery.
This is why a hybrid often runs its engine at a speed unrelated to road speed, a behaviour that feels unfamiliar but is central to the efficiency gain.
Motorway cruising offers little to recover
At a constant high speed there is almost no braking, so the recovery mechanism is idle and the battery contributes little beyond brief assistance.
The engine is already operating under steady moderate load, which is reasonably close to its efficient region without any help from the hybrid system.
The car is also carrying the extra mass of the battery and electric machine, so at that point the hybrid hardware is closer to a penalty than a benefit.
What this means for the buying decision
The saving scales with how much of a driver's mileage is urban, which is why fleets in city use adopted hybrids long before private buyers did.
A driver whose mileage is overwhelmingly motorway will see a much smaller difference, and the official test figure will overstate their likely experience.
Terrain matters too, since hilly routes offer repeated recovery on descents that a flat motorway route does not provide at all.