How fuel trip cost is calculated

The calculation has only two steps, which is why it is easy to check. First, work out how much fuel the trip needs: divide the distance by 100 and multiply by consumption in litres per 100 kilometres. Second, multiply that fuel amount by the price per litre. Everything else — traffic, hills, load, and driving style — affects the consumption figure you enter, not the formula itself.

Consumption is the figure most people get wrong. The number in a brochure is measured under test conditions. In real driving, short trips with a cold engine, stop-start traffic, air conditioning, roof racks, heavy loads, and high speeds all push consumption up. A useful habit is to record your own consumption: fill the tank, reset the trip meter, drive normally, fill again, and divide litres used by kilometres driven, multiplied by 100. That personal figure will beat any generic figure for your car and your driving.

Petrol price is the other variable. Prices differ between cities, suburbs, and even stations on the same road, and they move during the week. That is why this calculator asks you to enter a price instead of showing a stale one. Enter the price you actually expect to pay, and re-run the calculation if you plan to refuel in a cheaper area.