How fuel cost is calculated
The idea is simple: work out how much fuel the trip burns, then multiply by the price. The formula depends on how your car's economy is measured.
MPG: cost = distance ÷ MPG × price per gallon
L/100 km: cost = distance ÷ 100 × L/100 km × price per litre
MPG is "distance per fuel", so a higher number is better. L/100 km is "fuel per distance", so a lower number is better. The calculator converts everything to litres and kilometers in the background, so you can mix units: MPG with a price per litre, or miles with L/100 km.
A worked example
Three friends drive 250 miles to the coast and back, 500 miles in total, in a car that gets 28 MPG. Gas costs $3.20 a gallon. The car burns 500 ÷ 28 = 17.86 gallons, which costs 17.86 × $3.20 = $57.14, or $19.05 each. That is 11.4 cents a mile.
In metric: a 400 km trip at 6.5 L/100 km uses 400 ÷ 100 × 6.5 = 26 litres. At €1.75 a litre that is €45.50.
Fuel is only part of what driving costs. Tires, maintenance, insurance and depreciation add more per mile than fuel for most cars. If you split a trip with a friend's car, fuel plus a little for wear is fair.
MPG, UK MPG and L/100 km
A US gallon is 3.785 litres. A UK (imperial) gallon is 4.546 litres, about 20% bigger. That is why the same car shows a higher MPG figure in Britain. To convert:
| From | To | Formula |
|---|---|---|
| MPG (US) | L/100 km | 235.2 ÷ MPG |
| MPG (UK) | L/100 km | 282.5 ÷ MPG |
| MPG (US) | MPG (UK) | MPG × 1.201 |
| Price per US gallon | Price per litre | price ÷ 3.785 |
So 30 MPG in the US is 7.84 L/100 km, or 36.0 MPG in the UK. Switch the unit buttons above and the numbers convert for you.
Fuel and CO2
Burning one US gallon of gasoline releases about 8,887 grams (8.89 kg) of CO2, according to the EPA. A gallon of diesel releases about 10,180 grams. Per litre that is 2.35 kg for gasoline and 2.69 kg for diesel. European figures often quote about 2.31 kg per litre of petrol because the fuel blend differs slightly. The calculator uses the EPA numbers.
The 100-mile trip in the default example burns 3.33 gallons and puts out about 29.6 kg of CO2. The figure covers the tailpipe only, not the oil well, the refinery or the delivery truck.
Gas car or electric car
An electric car's economy is measured in kWh per 100 miles (or per 100 km). Most EVs use 25 to 35 kWh per 100 miles; big SUVs and pickups use 40 or more. At 30 kWh per 100 miles and $0.17 per kWh, a 500-mile trip needs 150 kWh and costs $25.50 at home, against $57.14 for the 28 MPG car above.
Public fast chargers often cost $0.40 to $0.60 per kWh, which can erase most of the saving on a road trip. Charging losses add another 10% or so. Put your real charging price in the calculator to compare fairly.
What gas costs today and how to spend less
The US average price for regular gasoline is published every week by the U.S. Energy Information Administration. Prices vary by more than a dollar a gallon between states, with California usually the highest. Diesel usually costs more than regular.
- Speed costs fuel. Most cars use noticeably more above 50 to 60 mph because air drag rises fast.
- Keep tires at the pressure on the door sticker. Low pressure raises rolling resistance.
- Remove roof boxes and racks when you do not need them.
- Smooth driving, with gentle acceleration and early braking, can save 15% to 30% at highway speeds, according to FuelEconomy.gov.
- Use premium fuel only if the maker requires it. If it is only "recommended", regular works.