Start with the correct gallon
The UK imperial gallon contains exactly 4.54609 litres. A US liquid gallon contains approximately 3.78541 litres. A UK figure of 45 MPG means 45 miles on 4.54609 litres, not 3.78541 litres. On imported car reviews and forums, this distinction matters. Our calculators deliberately use UK imperial MPG by default.
Why L/100km behaves differently
MPG is distance divided by fuel: higher is better. L/100km is fuel divided by distance: lower is better. Because one is the inverse of the other, you cannot translate a 10% MPG improvement directly into a 10% reduction in fuel spending. Work through litres consumed for the miles actually travelled.
A real-world cost example
At 45 UK MPG, a 100-mile drive uses approximately 10.10 litres. At 145p per litre that comes to about £14.65. At 35 UK MPG the same journey uses about 12.99 litres, costing about £18.83. Price changes and traffic can affect your bill as much as the official fuel-economy claim.
What changes the result
Motorway speeds, stop–start traffic, low tyre pressures, roof racks, cold starts and towing can change real fuel use. Use economy measured over comparable journeys rather than relying only on a brochure figure. Recalculate after a few full-tank measurements if your driving pattern changes.
Making apples-to-apples comparisons
When comparing petrol and diesel engines, use the same annual mileage and realistic MPG for each engine, then apply the corresponding pump price. An EV uses different units entirely: kWh/100 miles or miles/kWh. Avoid converting EV efficiency to MPG without carefully specifying the energy basis.
Questions people ask
Is 50 UK MPG good?
It depends on vehicle type, driving and conditions. Compare similar vehicles on the same measurement basis rather than treating one MPG threshold as universally good.
Why are official and real-world MPG different?
Standardised tests allow comparisons under controlled conditions; real roads, weather and driving styles vary substantially.
Check the official source
For current definitions and values, check the primary UK reference. Rules, rates and data sources can change after this guide was published.