Consider a one-way drive of 300 miles in a vehicle rated at 30 US MPG, with gasoline priced at $3.60 per US gallon. Because the distance and fuel standard use matching US units, the volume has an intuitive cross-check before the API normalizes it.
Keep the MPG standard explicit
Enter 300 miles, 30 US MPG, $3.60 per US gallon, one way, and currency $. Do not select UK MPG merely because both labels say MPG. A UK imperial gallon is larger and would describe a different efficiency for the same numeric value.
Cross-check the gallons
Distance divided by efficiency gives 300 miles ÷ 30 miles per US gallon = 10 US gallons. The API also converts this to about 37.854 litres and 8.327 UK gallons. Those alternate volumes describe the same fuel, not additional fuel to add.
Calculate total and cost per mile
Ten US gallons multiplied by $3.60 equals $36.00. Divide by 300 miles to get $0.1200 per mile. For a return on the same route, selecting round trip would double distance, fuel, and total cost while leaving cost per mile unchanged.