How the gas cost is calculated
cost = gallons × price per gallon
Everything uses US gallons (3.785 litres). If you're comparing with a UK figure, note that a UK gallon is about 20% bigger, so UK mpg numbers look higher for the same car.
Worked example
A 300-mile drive in a car that averages 25 mpg uses 300 ÷ 25 = 12 gallons. At $3.50 a gallon that's 12 × 3.50 = $42.00 one way, or $84.00 there and back. Split between four friends, the round trip is $21.00 each.
Getting a realistic mpg
- The EPA rating on the window sticker gives city, highway and combined mpg. For interstate driving, start from the highway figure.
- Roof boxes, a full load, towing, air conditioning and speeds well above 65 mph all cut real-world mpg. Knock off a few mpg if any apply.
- The most accurate number is your own: fill up, reset the trip meter, and divide miles driven by gallons at the next fill.
Prices change along the route
Gas prices vary a lot between states because of different fuel taxes, so a cross-country trip rarely has one price. Use an average of what you expect to pay, or run the calculator once per leg. This calculator never fetches prices — the price shown is only an example.
Not sure whether to drive at all? Compare it with flying in the driving vs flying calculator. Using km and litres? Try the Canada or Australia version.