How trip fuel cost is calculated
In the US, fuel economy is measured in miles per gallon, so the fuel a trip needs is the distance divided by mpg, and the cost is that fuel multiplied by the price per gallon. A 300-mile drive in a car that gets 30 mpg uses 10 gallons; at $3.50 a gallon that is $35.00. Most other countries measure consumption in liters per 100 kilometers, where a lower number is better. Then fuel = distance × L/100 km ÷ 100. A 500 km trip at 6.5 L/100 km uses 32.5 liters, which costs $58.50 at $1.80 a liter.
Use the toggle to switch between miles and mpg or kilometers and L/100 km. You can also choose a round trip, which doubles the distance, and split the cost between passengers, which is useful for carpooling and road trips with friends.
Finding your real fuel economy
Official ratings from the EPA or WLTP are a useful starting point, but real-world economy is often 10% to 20% worse, depending on how and where you drive. The most accurate figure comes from your own car: fill the tank, reset the trip meter, drive normally, then fill up again and divide the miles driven by the gallons added. Many modern cars also display an average on the dashboard. To convert between systems, 235.21 divided by mpg gives L/100 km, so 30 mpg is about 7.8 L/100 km.
What affects fuel use on a trip
Highway driving at a steady speed is usually more efficient than stop-and-go city traffic, but speed has a large effect: most cars use noticeably more fuel above about 50 to 60 mph (80 to 100 km/h) because air resistance rises quickly. Driving at 75 mph instead of 65 can cut economy by 10% to 15%. Heavy loads, roof boxes, towing, underinflated tires, cold weather and running the air conditioning all increase consumption. Mountainous routes cost more than flat ones. If you are planning a long trip, consider adding 10% to the estimate as a buffer.
Using the results
Fuel is only part of the cost of driving. The US Internal Revenue Service publishes a standard mileage rate each year for business travel that also covers depreciation, maintenance and insurance, and it is usually several times the fuel cost per mile. When comparing driving with flying or taking the train for a trip, include tolls, parking and the value of your time. When comparing cars, use the cost per mile or per kilometer from your results together with how far you drive each year. The difference between a 25 mpg car and a 40 mpg car over 12,000 miles at $3.50 a gallon is about $630 a year.
Frequently asked questions
How do I calculate gas cost for a trip?
Divide the distance by your car’s mpg to get gallons, then multiply by the price per gallon.
How do I convert mpg to L/100 km?
Divide 235.21 by the mpg figure. For example, 30 mpg is about 7.84 L/100 km.
Does speed affect fuel economy?
Yes. Above about 50 to 60 mph, fuel use rises quickly because of air resistance.
Should I include a round trip?
Choose “round trip” to double the distance if you are driving there and back.