Your real fuel economy, not the sticker figure
The EPA rating on a new car’s window sticker comes from standardized lab cycles. Your own mileage depends on your commute, your speed, the weather and the load you carry, and it is easy to measure precisely with nothing more than two fill-ups. This calculator turns those numbers into miles per gallon, converts them to the metric units used abroad, and puts a price on every mile.
How to measure it
- Fill the tank until the pump clicks off the first time, without topping up, and reset the trip odometer.
- Drive as you normally do, ideally until the tank is at least half empty.
- Fill up again to the first click and note the gallons pumped.
- Enter the trip miles and the gallons. Repeat over two or three tanks for a reliable average.
The formulas
- MPG = miles driven ÷ gallons used.
- Liters per 100 km = 235.215 ÷ MPG. The constant comes from 100 km per 62.137 miles and 3.785 liters per gallon.
- Kilometers per liter = MPG × 0.425144.
- Cost per mile = price per gallon ÷ MPG, and yearly cost = cost per mile × miles per year.
Worked example
You drove 350 miles and refilled with 12.5 gallons at $3.20.
- Fuel economy: 350 ÷ 12.5 = 28.0 mpg.
- Metric: 235.215 ÷ 28 = 8.40 L/100 km, or 11.90 km/L.
- Cost per mile: 3.20 ÷ 28 = $0.114, so 100 miles cost $11.43.
- Over 12,000 miles a year: $1,371 of fuel.
Same trip, different fuel used
| Gallons for 350 miles | MPG | Cost per mile | Cost per year |
|---|---|---|---|
| 8 | 43.8 | $0.073 | $878 |
| 10 | 35.0 | $0.091 | $1,097 |
| 12.5 | 28.0 | $0.114 | $1,371 |
| 15 | 23.3 | $0.137 | $1,646 |
| 20 | 17.5 | $0.183 | $2,194 |
The table shows why a few MPG matter more on a thirsty vehicle: going from 17.5 to 23.3 mpg saves more than $500 a year, while going from 35 to 43.8 mpg saves about $220.
How to improve your mileage
- Slow down on the highway. Aerodynamic drag rises quickly above 50 to 60 mph, so each extra 5 mph costs noticeably more fuel.
- Check tire pressure monthly and inflate to the figure on the driver’s door jamb, not the maximum printed on the tire.
- Remove roof racks and cargo boxes when you are not using them, and clear heavy items out of the trunk.
- Combine short errands. An engine that has not warmed up burns much more fuel per mile.
- Drive smoothly. Hard acceleration and late braking waste the energy you paid for at the pump.
Keep in mind that the cost figures here cover fuel only. To compare the full cost of owning a car, add insurance, maintenance, tires and depreciation, which together usually cost more than the gas itself.