How fuel economy is calculated
Fuel economy measures how far your car travels on a gallon of gas. The formula is simply mpg = miles driven ÷ gallons used. If you drove 300 miles and it took 12 gallons to refill the tank, your fuel economy is 300 ÷ 12 = 25 mpg.
How to use this calculator
The most accurate method: fill the tank, reset your trip meter, drive normally, then fill up again. Enter the miles driven from the trip meter and the gallons it took to fill the tank back up. The result is your real-world mpg. Once you know it, plug the number into the gas cost calculator to budget a trip. If your odometer reads in kilometres, convert it with the km to miles calculator.
Worked example
You fill up, drive 450 miles, and it takes 15 gallons to top off: 450 ÷ 15 = 30 mpg. Comparing tank to tank over time shows whether your driving habits, tire pressure or a service have changed your efficiency. For everyday spending, the unit price calculator works the same way for grocery value.
For a reliable number, average three or four full tanks rather than trusting a single fill. Always brim the tank to the same click of the pump — topping off differently is the biggest source of error, and a half-gallon discrepancy on a 12-gallon fill shifts the result by roughly 1 mpg. A typical modern sedan lands around 30–35 mpg combined, a compact SUV nearer 25–28 mpg. Pump readings in litres can be converted using the liters to gallons calculator.
Frequently asked questions
- How do I calculate miles per gallon?
- Divide the miles you drove by the gallons of fuel used. 300 miles on 12 gallons is 25 mpg.
- What is 300 miles on 12 gallons?
- 25 mpg. That is the distance divided by the fuel used.
- How do I measure my real mpg?
- Fill the tank, reset the trip meter, drive, then refill. Enter the trip miles and the gallons it took to fill up again.
- Why is my mpg lower than the sticker rating?
- City driving, cold weather, heavy loads and aggressive acceleration all lower real-world mpg below the official rating.
- How do I convert mpg to L/100 km?
- Divide 235.2 by your US mpg — 25 mpg becomes 235.2 ÷ 25 = 9.4 L/100 km. Watch the gallon too: an imperial (UK) gallon is about 20% larger than a US gallon, so UK mpg figures always look higher for the same car.