How to calculate fuel cost for a trip, per mile, and per year — and what really changes it

Fuel is the largest running cost of most cars after depreciation, and the arithmetic to see it per trip or per year is three numbers multiplied — once the units agree. This guide gives the formula, the conversions between the three economy units in use around the world, and the honest reasons your car never matches its official figure. The fuel cost calculator does the sums; the fuel economy converter the units.

The formula

Fuel used = distance ÷ economy; cost = fuel used × price. A 320 km trip in a car doing 6.5 L/100 km uses 20.8 litres; at £1.45 a litre that's £30.16. In imperial units: 200 miles at 40 mpg uses 5 UK gallons (22.7 litres). Per mile or per km, divide by the distance; per year, multiply by annual mileage — 12,000 miles at 40 mpg and £1.45/L (£6.59 per UK gallon) is about £1,980. Splitting a trip's cost among passengers is the same number divided by heads (the expense splitter handles the trip's other costs too).

mpg, L/100 km and km/L

The UK and US quote miles per gallon — with different gallons (a UK gallon is 4.546 L, a US gallon 3.785 L, so 40 UK mpg is 33.3 US mpg). Europe and most of the world quote litres per 100 km, which is a consumption figure (lower is better) rather than an economy figure (higher is better); Japan and some others use km per litre. Conversions: L/100 km = 282.5 ÷ UK mpg = 235.2 ÷ US mpg; km/L = 100 ÷ (L/100 km). The consumption form is the more useful for cost, because cost scales linearly with it — going from 5 to 6 L/100 km costs the same extra as going from 10 to 11, while the mpg scale compresses at the top and exaggerates small gains among efficient cars.

Why real consumption is worse than the brochure

Official figures come from standardised laboratory cycles (WLTP in Europe since 2017, EPA cycles in the US) that are more realistic than the old NEDC test but still gentler than real driving: no headwinds, mild temperatures, no roof box, no full car, no heater or air conditioning. Independent real-world data typically shows 10–20% worse economy than WLTP, more in winter and in town. Use the brochure to compare cars with each other; use your own tank-to-tank measurement — fill up, note the mileage, fill up again, divide — for the cost.

What actually changes the number

• Speed: air resistance rises with the square of speed; 80 mph uses roughly 25% more fuel per mile than 70, and 70 more than 60.
• Acceleration and braking: smooth driving that anticipates traffic saves 10–20% in town.
• Cold starts and short trips: the first few kilometres from cold use double; many short trips are the worst case.
• Tyre pressure: under-inflated tyres cost a few percent and are the cheapest fix.
• Weight and drag: a roof box costs 10–25% at motorway speed; 50 kg of junk in the boot, about 1%.
• Air conditioning: a few percent; open windows at speed cost about as much.

Comparing cars, and electricity

Per-mile fuel cost is the comparable number: (price per litre × L/100 km) ÷ 100 gives cost per km. A diesel doing 5 L/100 km at £1.55 costs 7.75p/km; a petrol doing 7 at £1.45, 10.2p/km. An electric car quoted at 17 kWh/100 km on home electricity at 28p/kWh costs 4.8p/km — and 15p/km on a 50p/kWh public rapid charger, which is the honest range for EVs (the electricity cost calculator does the kWh side). Fuel is only part of running cost — depreciation, insurance, tax, servicing and tyres usually exceed it — but it is the part you can change on any given trip. The speed-distance-time calculator estimates journey time for the same trip; the unit converter handles gallons.