Understanding fuel-cost maths
Distance times consumption times price.
The three-input formula, the L/100km vs MPG vs MPG-UK confusion, and the cost-per- kilometre number actually worth tracking.
The whole formula.
Fuel cost for a trip = distance × consumption × price. In SI: km × (L/100km) / 100 × price/L. In US: miles ÷ MPG × price/gal. Same operation; different units; same answer in different currencies.
cost = distance × consumption × price
The three consumption notations.
L/100km (Europe, Canada): litres of fuel per 100 km. Lower number = more efficient. Modern compact: 5-7 L/100km; SUV: 10-15. MPG (US): miles per US gallon. Higher = more efficient. Compact: 30-40 MPG; SUV: 18-22. MPG (UK): miles per imperial gallon (the UK gallon is bigger than the US). Same numerical idea but 20 % higher than US MPG for the same car. Conversion: MPG-UK = MPG-US × 1.201; L/100km = 235.21 / MPG-US.
A worked trip.
A 600 km trip in a car that does 6.5 L/100km at €1.85/L petrol. 600 × 6.5 / 100 × 1.85 = 600 × 0.0975 = €72.15. The middle number — €0.12 per km — is the useful per-kilometre figure for budgeting future trips. Once you know your car's cost per km at current fuel prices, distance is all you need to estimate.
600 km at 6.5 L/100km, €1.85/L
distance × (consumption/100) × price
Three multiplications.
600 × 6.5/100 × 1.85 = 72.15
= €72 for the trip
Speed affects consumption.
The catalogue MPG number is from a controlled test; real-world consumption varies. Driving at 130 km/h (81 mph) instead of 100 km/h increases consumption by 20-30 % — aerodynamic drag scales with the square of velocity. Climate control adds 5-10 %. Stop-and-go city driving roughly doubles consumption vs steady motorway. Use the catalogue number for comparing cars; use a measured per-kilometre number for real trip planning.
EVs make this calculator different.
Electric vehicles report kWh/100km (Europe) or miles/kWh (US) instead. The "fuel" price is the cost of electricity per kWh, which varies wildly with charging location: home overnight tariff (£0.05-0.15/kWh), public slow charger (£0.30-0.50), public fast charger (£0.70-1.00+). Home- charged Tesla Model 3: ~£0.03 per mile. Public-fast-charged same car: ~£0.20 per mile. Comparable to a thirsty petrol car. The vehicle's efficiency matters less than where you plug it in.
Tracking the real number.
Catalogue MPG is optimistic by 10-20 % in real-world use (WLTP figures closer than the older NEDC tests, but still). Tracking your own number is one tank of fuel: fill up, reset trip meter, drive normally, refill, divide. Repeat over five tanks for a stable average. The number you measure is more useful than any spec; once you have it, the rest of the calculator is just multiplication.