How to Calculate Fuel Cost for a Road Trip

Fuel cost comes down to three inputs: distance, your vehicle's fuel efficiency, and the price of fuel. In litres-per-100km terms, the formula is: (distance ÷ 100) × efficiency × price per litre. In miles-per-gallon terms, it's: (distance ÷ mpg) × price per gallon.

A 400 km trip in a car that uses 7 litres per 100 km, with fuel at $1.60 per litre, works out to (400 ÷ 100) × 7 × 1.60 = $44.80. The same logic applies in miles and gallons — only the units change, not the structure of the calculation.

Manufacturer efficiency figures are measured under controlled conditions and tend to undercount real-world consumption by 10–20%, especially in city driving, cold weather or with a loaded roof rack. If you have a few recent fill-up receipts, calculating your own actual litres-per-100km from odometer readings gives a far more reliable number than the sticker figure.

Fuel price varies by region and by day, so pull a current local price rather than reusing a number from a previous trip — a few cents per litre compounds quickly over a long distance.

For a return trip shared evenly, double the one-way cost and divide by the number of people paying. A cost that doesn't split evenly (say, one person is driving further before dropping others off) needs to be calculated per-leg rather than as one flat trip total — add up each segment's own distance × efficiency × price before dividing.