Calculate fuel use before calculating trip cost
For L/100 km, fuel used equals distance in kilometers multiplied by consumption and divided by 100. Kilometers per liter and US or UK miles per gallon are converted to L/100 km first. A round trip doubles the entered one-way distance. Fuel cost then equals liters used multiplied by the price per liter.
Keep gallon and mileage definitions explicit
A US liquid gallon is 3.785411784 liters, while a UK imperial gallon is 4.54609 liters. US mpg and UK mpg therefore describe different fuel economies even when the number is the same. Price per gallon is converted using the matching gallon definition before cost is calculated.
Use a realistic trip average and treat the result as an estimate
Actual fuel use varies with traffic, speed, idling, route, elevation, weather, cargo, tire pressure, engine condition, and driving style. Additional costs and contingency are split evenly among the entered travelers. The currency selector only labels values and does not perform exchange-rate conversion.