Method
Consumption = fuel liters ÷ kilometers × 100. US mpg = 235.214583 ÷ consumption. Distance and fuel are converted to metric before calculating.
Worked example
500 km using 40 L gives 8 L/100 km, 12.5 km/L and approximately 29.401823 US mpg.
Assumptions and limits
For a tank-to-tank measurement, fill to the same level, track distance, then use the fuel added at the next fill. US and imperial gallons differ; use the unit on your fuel measurement.
Odometer readings and cost
Distance = odometer end − odometer start. With a fuel price per liter or gallon, fuel cost = fuel used × price, and cost per mile or per kilometer divides that cost by the distance. Conversions use the exact definitions 1 mi = 1.609344 km, 1 US gal = 3.785411784 L and 1 imperial gal = 4.54609 L.
Worked example: odometer
Readings of 12,000 and 12,300 miles on 10 US gallons give 300 ÷ 10 = 30 US mpg, which is 7.84 L/100 km or 36.03 imperial mpg. At 3.50 per gallon the fuel cost 35, or about 0.117 per mile and 0.072 per kilometer.