Fuel Cost Calculator

Converting MPG to L/100km

The UK uses miles per gallon (MPG) for fuel economy, while almost everywhere else in the world (including the EU and on many UK car spec sheets) uses litres per 100 km. Converting between them isn't intuitive because they measure things differently — MPG is a 'distance per fuel' measure where higher is better; L/100km is a 'fuel per distance' measure where lower is better. The conversion formula: divide 282.5 by your MPG to get L/100km, or divide 282.5 by your L/100km to get MPG. (The 282.5 figure comes from converting gallons to litres, factoring in that UK gallons are 4.546 litres and 100 km is 62.137 miles, so 100 km / 1 UK gallon = 22.04 mpg, and (100 × 4.546 / 1.609) ≈ 282.5.) A worked example: a car achieving 40 MPG (UK) equates to 282.5 ÷ 40 = 7.06 L/100 km. A modern small petrol car at 55 MPG = 5.14 L/100 km. A thirsty SUV at 25 MPG = 11.3 L/100 km. An efficient diesel hatchback at 65 MPG = 4.35 L/100 km. Watch for US gallons: a US gallon is 3.785 litres (smaller than the UK gallon), so US MPG figures are about 17% lower than UK MPG for the same car (a car returning 40 MPG UK would be advertised as 33 MPG US — same actual efficiency, different gallon). Most published MPG figures are 'combined' cycle from official tests (WLTP since 2018 in the UK, replacing the optimistic NEDC). Real-world economy is typically 10–20% worse than the WLTP figure depending on driving style, conditions, and journey type — so a quoted 50 MPG often delivers 40–45 MPG in practice.

Annual Fuel Cost

Working out your annual fuel cost is one of the highest-value calculations when choosing between car models, because the difference between an efficient and an inefficient car compounds into thousands of pounds over a typical ownership period. To find your annual fuel cost: multiply your annual mileage by your fuel cost per mile (which is fuel price per litre ÷ MPG × 4.546). A worked example: an average UK driver covering 7,400 miles a year in a car returning 40 MPG with petrol at £1.45/litre spends 7,400 × £1.45 × 4.546 ÷ 40 = £1,219/year. The same driver in a 50 MPG car pays £976 (£243 saved), and in a 30 MPG car £1,625 (£406 more). Over a typical 6-year ownership period, the difference between a 50 MPG and 30 MPG car for that driver is over £3,900 — easily enough to offset a higher purchase price for the more efficient model. Higher-mileage drivers see proportionally larger savings: a 20,000-mile-a-year driver doubles those numbers. Electric vehicles change the calculation significantly (see below), and diesel typically gives 15–25% better MPG than petrol for the same model but at a higher per-litre cost and with more complex maintenance. The 'total cost of ownership' (purchase price minus resale value, plus running costs over the period) is the right way to compare options, but fuel cost is the single biggest factor over a long ownership period at typical UK fuel prices. This calculator handles the per-mile and annual cost maths, and lets you compare two vehicles side by side.

Electric Vehicle Equivalent

Electric vehicles use a different efficiency unit — kilowatt-hours per 100 km (kWh/100 km) or miles per kWh — that maps to fuel economy differently, and the running-cost comparison usually heavily favours EVs. A typical EV uses 15–20 kWh/100 km depending on size and conditions, with efficient compact EVs (Tesla Model 3, Hyundai Kona EV) at 13–16 kWh/100 km and larger SUVs at 18–25 kWh/100 km. At the UK domestic electricity rate of around £0.28/kWh (Ofgem cap, mid-2026), that's £4.20–5.60 per 100 km — versus around £10.50 for a petrol car at 7 L/100 km and £1.50/litre. So home-charged EVs typically cost less than half the per-mile fuel of a petrol equivalent. Off-peak EV tariffs (Octopus Go, Intelligent Octopus, etc.) drop the rate to £0.07–0.10/kWh overnight, taking the per-100-km cost to £1.20–2.00 — about a fifth of the petrol equivalent, an enormous saving for high-mileage drivers. Public rapid charging is more expensive (£0.70–0.85/kWh on some networks), so EVs running mostly on public chargers can cost similar to or even more than petrol per mile — the EV economic case strongly depends on having home charging or off-peak tariffs available. The break-even purchase-price premium for an EV depends on annual mileage, electricity tariff, and how long you keep the car. For an average UK driver charging at home with an off-peak tariff, EV running costs are typically £600–1,000/year less than petrol, plus zero road tax (until April 2025), lower servicing costs (fewer moving parts, no oil changes), but higher insurance. Compare total cost of ownership for an honest like-for-like comparison.

Reducing Fuel Costs in Practice

Practical fuel-saving techniques: check tyre pressure monthly (every 10% under-inflation adds approximately 2% fuel consumption). Remove roof boxes when not in use (add 10-20% drag at motorway speed). Air conditioning only when necessary above 80km/h — below this, opening windows is more efficient. Coast in gear (engine braking) to traffic lights rather than freewheeling in neutral — modern engines cut fuel entirely when decelerating in gear. Fuel price apps show the cheapest nearby stations for