Fuel-cost formula
gallons = miles ÷ MPG
cost = gallons × price per gallon
cost = gallons × price per gallon
Annual cost uses the same math with your yearly mileage. The estimate is most useful when comparing vehicles, commutes or changes in fuel economy.
Why real cost changes
Traffic, idle time, towing, weather, tire pressure, speed and driving style can move real MPG away from the value you enter. Fuel prices also change over time.