Fuel Cost Calculator
Trip cost, MPG, and the conversion to L/100km in one fuel cost calculator. It also shows why MPG makes a 15-to-20 upgrade look smaller than it is.
Trip cost
- Fuel used Over 300 miles.
- 10.71 US gallons
- Cost per mile
- $0.12
- Cost per km
- $0.08
- Annual fuel cost At 12,000 miles a year.
- $1,478.57
- Economy in every unit A US gallon is 3.785 liters and an imperial gallon is 4.546, so UK MPG figures are about 20% higher than US ones for the same car. Comparing them directly is the standard transatlantic error.
- 28 MPG (US) 33.63 MPG (imp) 8.4 L/100km 11.9 km/L
- Against a vehicle at 38 MPG 112.78 US gallons difference over the year.
- $389.10 a year saved
- Why MPG misleads Both are a 5 MPG improvement and the first saves three and a half times as much fuel. MPG is distance per fuel, so it is inversely related to what you spend. That is why the rest of the world quotes liters per 100 km, where the scale is linear in cost.
- 15 to 20 MPG saves $690.00; 30 to 35 saves $197.14
- Fill-ups for this trip Assuming a 14-gallon tank, which is typical for a mid-size car.
- 0.8 tanks
| MPG | Fuel a year | Cost a year | vs now |
|---|---|---|---|
| 15 | 800 | $2,760.00 | $1,281.43 |
| 20 | 600 | $2,070.00 | $591.43 |
| 25 | 480 | $1,656.00 | $177.43 |
| 30 | 400 | $1,380.00 | -$98.57 |
| 35 | 343 | $1,182.86 | -$295.71 |
| 40 | 300 | $1,035.00 | -$443.57 |
| 50 | 240 | $828.00 | -$650.57 |
- The price is per the unit the economy is quoted in: per gallon for MPG, per liter for L/100km. Mixing them is the usual way this calculation goes wrong by a factor of about four.
- Real economy is well below the sticker figure in city driving, cold weather and at speeds above about 60 mph, where aerodynamic drag dominates. Treat any single number as an average over a mixed year.
How the Fuel Cost Calculator works
Fuel cost is distance divided by economy times price, and the only difficulty is that the world measures economy two incompatible ways.
MPG is distance per unit of fuel: miles per gallon. Liters per 100 km is fuel per unit of distance. MPG misleads about money because it is the reciprocal of consumption, so equal steps in MPG are not equal steps in fuel used. The consequence is counterintuitive and well documented as the "MPG illusion": replacing a 15 MPG truck with an 18 MPG one saves more fuel than replacing a 30 MPG car with a 50 MPG one. If you want to reduce fuel use, the biggest wins are at the bottom of the range, not the top. This is why most of the world quotes liters per 100 km, where the number is linear in both fuel and cost, and why US fuel labels have carried gallons-per-100-miles alongside MPG since 2013.
Two other things the sticker figure does not tell you. Real-world economy runs well below the rated figure in city driving, in cold weather (where a short trip can be 20% worse because the engine never reaches operating temperature) and at speed. Aerodynamic drag rises with the square of velocity and the power needed to overcome it with the cube, so economy falls sharply above about 60 mph. Driving at 75 instead of 65 typically costs 10 to 15%.
And fuel is only part of what a mile costs. The IRS standard mileage rate, which is meant to approximate the full cost of operating a vehicle, has run around 65 to 70 cents a mile in recent years. Fuel at 28 MPG and $3.45 a gallon is about 12 cents of that. The rest is depreciation, insurance, maintenance and tires. That is why fuel economy, while the most visible running cost, is rarely the largest one.
Fuel Cost Calculator FAQ
How do I calculate the cost of a trip?
Divide the distance by the fuel economy to get the fuel used, then multiply by the price. 300 miles at 28 MPG is 10.7 gallons, and at $3.45 a gallon that is $36.96. Make sure the price is per the same unit the economy is quoted in.
How do I convert MPG to liters per 100 km?
Divide 235.215 by the MPG figure. 28 MPG is 8.4 L/100km. They are reciprocals, so higher is better for one and lower is better for the other. Use 282.481 instead if the MPG figure is in imperial gallons.
Why is UK MPG higher than US MPG?
Different gallons. An imperial gallon is 4.546 liters and a US gallon is 3.785, so the same car gets about 20% more miles from an imperial gallon. A car rated 40 MPG in Britain is roughly 33 MPG in America.
Is going from 15 to 20 MPG the same saving as 30 to 35?
No, and it is not close. Over 12,000 miles the first saves 200 gallons and the second saves 57. MPG is the reciprocal of consumption, so equal steps are not equal savings. That is why the biggest fuel reductions come from improving the worst vehicles, not the best.