Multiply the number of gallons you need times the average gas price to get your total fuel cost. For example, if gas is about $3.50 a gallon in your area and you need 20 gallons to drive your trip distance, you'll pay $75 for gasoline.hen divide this mileage figure by your car's MPG rating to find out how many gallons of fuel you'll need on your trip. For instance, if you plan to drive 500 miles in a car that gets 25 MPG, you'll need about 20 gallons of gasoline.