With miles per gallon, fuel needed = distance ÷ MPG. With liters per 100 km, fuel needed = distance × L/100km ÷ 100. Multiply the fuel by the price to get the total cost, then divide by the number of people. Tick round trip to double the distance.
Example: 300 miles at 30 MPG needs 10 gallons; at $3.50 a gallon the trip costs $35.00.
It is an estimate. Real fuel economy changes with speed, load, weather and traffic; highway driving at high speed can use noticeably more fuel than the rated figure. Use your own average from the dashboard for best results.
Divide 235.21 by the MPG (US gallons). 30 MPG is about 7.84 L/100 km. See the MPG to L/100km converter.
See also: MPG to L/100km · Km to Miles · Pace Calculator