What this guide helps you decide
Fuel budgets fail quickly when distance, economy, and price use incompatible units. Litres per 100 kilometres is a consumption measure, while miles per gallon is an economy measure; one increases as efficiency worsens and the other decreases.
Keep the original unit labels attached to every input. US and imperial gallons are different volumes, so a bare ‘mpg’ value is incomplete unless its convention is known.
Normalize the trip before pricing it
For L/100 km, fuel used = distance in kilometres × consumption ÷ 100. For an economy measure, fuel used = distance ÷ distance per unit of fuel after applying the registered unit conversion.
- Choose one distance unit and include detours or local driving in the route estimate.
- Select the exact fuel-economy convention printed for the vehicle.
- Convert fuel use to the unit in which the price is quoted.
- Apply a transparent contingency rather than altering the vehicle's measured efficiency without explanation.
Worked scenario: metric consumption
A 640 km route is planned in a vehicle rated at 7.2 L/100 km, with fuel entered at $1.74 per litre.
- Fuel used: 640 × 7.2 ÷ 100 = 46.08 litres.
- Fuel cost: 46.08 × 1.74 = $80.1792, displayed as $80.18.
- A 10% route-and-idling contingency would be shown separately as about $8.02.
Outcome: The base estimate is 46.08 L and $80.18. Keeping contingency separate preserves the meaning of the entered consumption rate.
Trip-input checklist
- Confirm km versus miles for the route.
- Confirm L/100 km, km/L, US mpg, or imperial mpg.
- Match the fuel-price unit to litres or the correct gallon.
- Add route contingency as a labeled scenario.
- Do not treat an official test rating as a guarantee for traffic or weather.
Limits and responsible use
- Actual consumption varies with speed, load, temperature, terrain, traffic, tyre condition, and driving style.
- The calculator uses a user-entered price and is not a live fuel-price service.
Authoritative references
These links support the definitions, conventions, or safety boundaries used in this guide. CalculatorToolset wrote the explanation and example independently.
- Fuel Economy GuideU.S. Department of Energy and U.S. Environmental Protection Agency
- NIST Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology
Frequently asked questions
Why are US and imperial mpg different?
The gallon volumes differ. A US gallon is smaller than an imperial gallon, so the same vehicle and route produce different numeric mpg values.
Should I add the return journey twice?
Enter total route distance once. If the route value already includes both directions, do not double it again.