Trip fuel cost: start with the full distance, not the refill bill
The price of filling a tank does not directly measure one trip: much of that fuel may remain afterwards. A better estimate connects route distance to expected consumption and then prices that quantity.
Keep the three quantities consistent
With consumption in litres per 100 kilometres, expected litres = distance in kilometres ×consumption ÷100. Multiply those litres by the price per litre you supply to estimate cost.
Do not put a kilometres-per-litre value into a litres-per-100-kilometres field; they describe inverse relationships. Include the return leg in total distance when needed, without doubling it a second time.
Expected consumption is not guaranteed
FuelEconomy.gov explains that standardised tests help comparisons, but driving, location, vehicle condition and other factors can change actual consumption. A published rating is not a promise for every journey.
Observations from comparable previous trips can help form an assumption. Compare two consumption scenarios to expose uncertainty rather than presenting one estimate as a final bill.
References: FuelEconomy.gov — Your mileage will vary
What the estimate leaves out
Fuel cost is not the total cost of travel or vehicle ownership. Parking, tolls, maintenance and other expenses are not included in this calculation.
Dividing by passenger count gives only an equal arithmetic share. Agree what to split and use a price with a known source and date; do not label an assumed input as today’s fuel price.
Worked example: a return trip with 210-kilometre legs
Each direction is 210 kilometres, so the total is 420. At an assumed 7.2 L/100 km and a manually supplied price of 2.5 per litre, expected fuel is 30.24 litres and cost is 75.6. Split equally between 3 people, that is 25.2 each.
If a second scenario uses 8 L/100 km, expected fuel becomes 33.6 litres and cost 84, an increase of 8.4. These are illustrative assumptions, not live prices or a guarantee of any vehicle’s performance.