Fuel economy labels in the UK still default to miles per gallon, even as official test data and manufacturer brochures increasingly quote litres per 100 km alongside it. The two describe exactly the same efficiency, but they behave very differently as numbers. MPG rewards small improvements at the top end of the scale far less than the same-sized jump at the bottom end. If you're comparing two cars, or deciding whether a diesel is still worth the extra outlay, knowing which unit to trust can change your conclusion — and your annual fuel bill.
Why two numbers for the same thing can mislead you
MPG measures distance covered per unit of fuel; L/100km measures fuel used per fixed distance. Because one is essentially the inverse of the other, equal differences in MPG do not translate into equal differences in fuel actually burned. A car improving from 20 to 30 MPG uses noticeably less fuel over a given journey than a car improving from 60 to 70 MPG, even though both gains look like the same '10 more MPG' on a spec sheet.
Run the numbers and the gap is stark. Moving from 20 to 30 MPG cuts consumption from roughly 14.1 to 9.4 litres per 100 km — a saving of about 4.7 litres for every 100 km driven. Moving from 60 to 70 MPG only trims consumption from about 4.7 to 4.0 litres per 100 km, a saving of under 0.7 litres. The MPG improvement is identical on paper; the real-world fuel saved is roughly seven times smaller in the second case.
This matters most when comparing an efficient petrol car against an even more efficient hybrid, or when a dealer highlights a headline MPG figure that sounds dramatically better than a rival's. At the top end of the scale, MPG comparisons flatten out and can exaggerate differences that barely register in litres, miles, or pounds spent.
The conversion, and why it isn't a straight line
The standard conversion for the UK's imperial gallon is L/100km equals roughly 282.5 divided by MPG, and MPG equals roughly 282.5 divided by L/100km. Because this is a division, not a simple multiplication, plotting one against the other produces a curve rather than a straight line — steep at low MPG values, then flattening out beyond about 50 MPG.
This is one reason the Vehicle Certification Agency's official WLTP fuel consumption and CO2 database now lists both units for every new car, and why DESNZ's own reporting on transport energy increasingly favours litres per 100 km for consistency across fuel types and international comparisons. MPG remains useful for quick mental arithmetic on a UK trip, but it is not the unit to reach for when ranking multiple cars precisely.
Turning MPG into pounds and pence
Recent AA Fuel Price Report and RAC Fuel Watch figures have generally put average UK unleaded petrol in the region of 140–145p per litre, with diesel typically a few pence higher. Using a representative 145p/L and a typical 10,000-mile annual mileage, a car doing 40 MPG uses around 1,140 litres a year, costing roughly £1,650. The same driver in a 50 MPG car spends around £1,320 — a saving of about £330 simply from a 10 MPG improvement at the lower end of the scale.
Apply the identical 10 MPG jump higher up the scale and the picture changes. Going from 60 to 70 MPG on the same mileage takes annual fuel spend from roughly £1,100 to around £940 — a saving of about £160, less than half the cash benefit of the 40-to-50 MPG jump, despite an identical MPG improvement. This is the practical, pounds-and-pence version of the same non-linear relationship.
The lesson for anyone cross-shopping cars, or weighing up whether a mild hybrid is worth a premium over a comparable petrol model, is to convert headline MPG figures into an estimated annual fuel cost using current pump prices before drawing conclusions. A DVSA MOT history or manufacturer WLTP figure is a starting point, not the final answer.
Real-world driving versus WLTP lab figures
Official WLTP figures are produced under standardised laboratory conditions and are useful for comparing like with like between models, but independent testing bodies and consumer research have repeatedly found real-world fuel consumption running higher than the lab figure, often by somewhere in the region of 10 to 20 percent depending on driving style, road type and payload. This gap tends to be proportionally similar whether the car is quoted in MPG or L/100km, so it doesn't change which unit is more useful for comparison — it simply means neither number should be treated as a guarantee.
For a realistic annual cost estimate, it's worth applying a modest real-world uplift to the official figure rather than taking the brochure number at face value, particularly for cars that spend a lot of time in stop-start urban traffic rather than at a steady motorway cruise.
The takeaway
MPG and litres per 100 km measure the same underlying efficiency, but because one is the inverse of the other, identical MPG improvements do not deliver identical fuel or cost savings — the benefit shrinks sharply as MPG rises. When comparing cars, especially efficient ones, convert headline MPG figures into litres per 100 km or, better still, into an estimated annual fuel cost using current AA or RAC average pump prices, and treat official WLTP numbers as a comparison tool rather than a real-world promise.
