The short answer
The MPG figure in a car's specification is a laboratory result. Since September 2017 new cars have been measured under a procedure called WLTP, which the Vehicle Certification Agency describes as a closer representation of real-world fuel consumption than the test it replaced, but still a figure obtained under specific test conditions. It is a fair way to compare one car with another. It is not a promise about your commute.
What you actually get depends on how and where you drive. The official driving guidance puts numbers on the big ones: speed, roof loads, tyre pressure and harsh acceleration. Those matter far more than the choice of fuel grade.
What the official figure measures
WLTP stands for the Worldwide Harmonised Light Vehicle Test Procedure. It replaced the older NEDC test for new cars in September 2017 and became mandatory for all new petrol and diesel cars by September 2018. Every car is run through the same laboratory cycle so that the figures can be compared like for like. That is the point of the test, and it is why the same model always shows the same number regardless of who is driving.
For a used car, the WLTP figure (or, for cars first sold before 2017, the older NEDC figure) is the starting point and nothing more. Two identical cars with two different owners will show two different fuel bills. The things that move the bill are below.
| What you do | Effect on fuel use (official guidance) | Source |
|---|---|---|
| Drive at 75mph instead of 60mph | Around 18% more fuel | Vehicle Certification Agency |
| Carry a roof box at 62mph | Around 22% more | Vehicle Certification Agency |
| Carry a roof box at 75mph | Around 39% more | Vehicle Certification Agency |
| Leave empty roof bars on at motorway speed | Around 7% more | Vehicle Certification Agency |
| Fill with E10 rather than E5 petrol | Around 1% less economy, unlikely to be noticed | GOV.UK |
Two more items in the same guidance carry no percentage but the same message: under-inflated tyres make the engine work harder, and harsh acceleration is particularly bad for fuel use and wears the engine and tyres faster.
Why your car does not match the brochure
The test cycle has no headwind, no hills, no cold starts on a frosty morning and no school run. Your week has all of them. Short trips from cold use more fuel per mile than one long run, because the engine spends its first minutes running rich. Town traffic means repeated acceleration from a standstill, which is the costliest thing an engine does. A boot full of weight, a roof box left on after the holiday and tyres a few pounds under pressure all add up.
None of this means the official figure is wrong. It means the official figure is a comparison tool. Use it to choose between cars; use your own fill-ups to learn what the car you own really does.
How to work out your real MPG
- Fill the tank to the first click and note the mileage. Reset the trip counter if the car has one.
- Drive normally for a full tank. One tank is a rough answer; three or four tanks average out the weather and the traffic.
- Fill to the first click again and note the litres and the miles. Miles divided by litres gives miles per litre; your car's handbook or any online converter turns that into miles per imperial gallon, which is the gallon the UK figure uses.
- Compare with the official figure, then forget it. The gap tells you about your driving, not about a fault. Compare your own tanks with each other from then on.
- Watch the mileage record when buying. The mileage recorded at each MOT since 2005 is on GOV.UK check MOT history; a car that has done mostly long motorway miles tends to have been kinder to itself, and to its fuel bill, than one with the same total in town.
What this does not mean
Nothing here tells you what any one car will cost to fuel; that depends on your routes, your right foot and the pump price on the day. The percentages in the table are the official guidance's typical figures, not measurements of your car. This article does not advise on fuel brands, fuel grades beyond what GOV.UK says about E10, or on any product. The full text is in the Vehicle Certification Agency's fuel-efficient driving tips and GOV.UK's E10 guidance.
Frequently asked questions
The Worldwide Harmonised Light Vehicle Test Procedure. It is the laboratory test that has produced the official fuel consumption and CO2 figures for new cars since September 2017, replacing the older NEDC test.
Because the official figure is measured in a laboratory under fixed conditions so that cars can be compared fairly. Cold starts, short trips, town traffic, speed, roof loads and tyre pressure all add fuel use that the test cycle does not include.
Slightly. GOV.UK says E10 can reduce fuel economy by around 1%, which is unlikely to be noticeable in everyday driving, and that driving style, tyre pressure and roof racks matter far more.
Speed and roof loads, on the official figures: 75mph instead of 60mph uses around 18% more fuel, and a roof box adds around 22% at 62mph and 39% at 75mph. Tyre pressure and smooth acceleration come next.
Sources and method
This article contains no MOT statistics. Every percentage and date is quoted from the Vehicle Certification Agency and GOV.UK pages listed under Sources, as they read on 26 October 2026. The MOT reliance notice below applies to the MOT history check this article recommends, not to any figure in it.
- Vehicle Certification Agency: the Worldwide Harmonised Light Vehicle Test Procedure (WLTP)
- Vehicle Certification Agency: fuel-efficient driving tips
- GOV.UK: E10 petrol explained
- GOV.UK: Check the MOT history of a vehicle
About these figures. Vehicle Certification Agency and GOV.UK content reproduced in summary under the Open Government Licence v3.0. SortedCars does not test vehicles or sell fuel; the links above go to the official guidance. Where MOT statistics appear on this site they are computed from the DVSA anonymised MOT results dataset. No figure is shown for any group with fewer than 500 tests. Test class 4 only, first tests only; retests are excluded. Pass rates describe fleets, not individual machines: they reflect how cars were driven, maintained and used as well as how they were built, and beyond recorded mileage none of that is in the data. A well-kept example of a model with a weak average can still pass; a neglected example of a strong one can still fail. Alias spellings of a make are counted under one make name and all trims of a model under one model name, using SortedCars' grouping tables.
This article is general information drawn from published test records. It is not professional, mechanical or legal advice, and an MOT pass or a clean MOT history does not guarantee that a car is roadworthy today. If you are unsure about a car, have it inspected by a qualified mechanic. You can check any vehicle's MOT history free on the official GOV.UK MOT history service.
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