Two numbers measuring different things
The figure on a vehicle’s documentation and the figure a driver calculates at the pump are both real, and they are answers to different questions. The official number comes from a defined procedure run under controlled conditions so that any two vehicles can be compared on the same basis. The pump number is what happened on your roads, in your traffic, with your right foot.
A test that produced a personalised prediction would be useless for comparison, because no two people drive alike. A test that produces comparability necessarily strips out everything variable, and everything variable is most of what determines real consumption.
Repeatability is the whole design constraint
For a published figure to mean anything, the same vehicle tested twice must produce the same result, and two vehicles tested in different places must be measured identically. That forces the test indoors, onto a rolling road, at a controlled temperature, following a prescribed sequence of speeds, with ancillary systems in defined states.
Every one of those controls removes a real-world cost. Ambient temperature affects engine warm-up and battery performance. Wind and gradients are absent. Air conditioning, heated seats and lights draw power that a laboratory sequence may not represent as your use does. None of this is concealed; it is the published methodology.
The test cycles have been revised, and they are still tests
Regulators in several markets replaced older laboratory cycles with newer, longer and more dynamic ones, precisely because the gap between published and achieved figures had become embarrassing. Some markets added on-road emissions testing alongside. The revisions narrowed the gap and did not close it, and they were not adopted simultaneously everywhere, so figures from different periods and jurisdictions are not directly comparable.
This matters when comparing a used vehicle advertised with an older figure against a newer one quoted under a revised standard. They are not the same measurement, and the apparent difference between the cars may be mostly a difference between test regimes.
Hybrid and plug-in vehicles complicate the comparison further, because their published figures depend on assumptions about how much of a journey runs on stored electricity. A plug-in driven short distances and charged diligently can beat its official figure comfortably; the same car used for long journeys without charging will not come close. For those models more than any other, the published number describes a usage pattern rather than a vehicle.
What actually decides consumption on the road
The physics is unforgiving. Aerodynamic drag rises with the square of speed, and the power needed to overcome it rises with the cube, which is why sustained high-speed cruising is disproportionately expensive. Rolling resistance depends on tyre construction and inflation. Every acceleration converts fuel into kinetic energy, and every application of the brakes throws that energy away as heat unless the vehicle can recover some of it.
Cold starts are the other large item and the least visible. An engine below operating temperature runs richer and lubricates worse, so a series of short journeys can consume far more per kilometre than a single longer one covering the same distance. Anyone whose driving is mostly short trips should expect to sit well away from the official figure permanently.
Then there is load and shape: a roof box, a full boot, an underinflated tyre, a towed trailer. Each is small on its own and they accumulate.
Measuring your own reliably
Trip computers estimate rather than measure, and their errors are usually consistent in one direction for a given vehicle, which makes them useful for comparison but not for absolute figures. The reliable method is brim to brim: fill completely, record the distance to the next complete fill, and divide.
One tank tells you very little, because the mix of journeys varies. Several tanks across different seasons gives a genuine baseline, and a baseline is what makes it possible to notice a change — a developing fault, a dragging brake, or simply a tyre that has been losing pressure for a month.
What the divergence means for a purchase decision
Published figures remain useful for what they were built for: ranking vehicles against one another. A car with a substantially better official figure than another will usually be better in practice too, even if neither achieves the number.
What they should not be used for is budgeting, because the size of the gap depends on how the car will actually be used. A vehicle bought for a long motorway commute and one bought for short urban journeys will diverge from the same official figure by very different amounts, in opposite directions from each other.
