The fuel cost formula
Fuel cost is the amount of fuel a trip burns times the price of fuel. In US units (miles per gallon):
gallons used = distance ÷ MPG
cost = gallons used × price per gallon
In metric units (litres per 100 km):
litres used = distance × ( L/100km ) ÷ 100
cost = litres used × price per litre
The two efficiency systems are inverses: L/100km = 235.215 ÷ MPG.
Worked example
A 300-mile trip in a car that does 30 MPG, with gas at $3.50 a gallon:
Gallons: 300 ÷ 30 = 10 gallons.
Cost: 10 × $3.50 = $35.00.
Round trip? Double the distance — a 300-mile one-way trip becomes 600 miles and $70 there-and-back. Enter the full distance you will actually drive.
Use your real efficiency
The window-sticker MPG is a lab figure. Real economy depends on speed, traffic, terrain, load and weather. For the best estimate, use the average you have actually observed (miles driven ÷ gallons filled). Fuel prices also swing, so check the current pump price near your route.