About the Mileage Calculator
A mileage calculator works out fuel economy from distance driven and fuel used, along with the cost per mile. It tells you what a vehicle actually achieves rather than what its official rating claims, and what a journey genuinely costs.
The formula
MPG = miles / gallons; Cost per mile = fuel price / MPGMiles per gallon measures distance per unit of fuel — higher is better. Litres per 100 km, used in most of the world, is the inverse: lower is better.
How to use this calculator
- 1Enter your Distance Driven (Miles). The field starts at
300, which you can overwrite. - 2Enter your Fuel Used (Gallons). The field starts at
12, which you can overwrite. - 3Enter your Fuel Price ($/Gallon). The field starts at
3.5, which you can overwrite. - 4Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Distance Driven (Miles) | 300 |
| Fuel Used (Gallons) | 12 |
| Fuel Price ($/Gallon) | 3.5 |
Result
25.0 MPG
Total Cost: $42.00
Cost/Mile: $0.14
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Real-world economy usually falls below the official rating, often by 10 to 20 percent, because test cycles cannot reproduce actual driving. Short trips are the worst case: a cold engine can consume double its warm rate for the first few miles, which is why urban stop-start driving produces the lowest figures.
The relationship between MPG and fuel cost is not linear, and this is where fuel economy comparisons mislead. Improving from 15 to 20 MPG saves more fuel over 10,000 miles than improving from 40 to 50 MPG, because the low-efficiency vehicle was burning so much more to begin with. Cost per mile, or litres per 100 km, makes this visible in a way MPG does not.
Things worth knowing
- Fill to full, reset the odometer, then fill to full again to measure real economy accurately.
- Steady speeds around 50 to 60 mph are typically most efficient. Economy drops sharply above 70.
- Correct tyre pressure improves economy by around 3 percent, and it is the cheapest available improvement.
- Roof boxes and racks can cut highway economy by 10 to 25 percent through aerodynamic drag.
- Litres per 100 km compares fuel consumption more intuitively than MPG, which is why most of the world uses it.
Frequently asked questions
How do I calculate my fuel economy?+
Divide the miles driven by the gallons used. Fill the tank completely, reset the odometer, drive, then fill completely again — the second fill tells you exactly how much you used.
Why is my real economy worse than the official rating?+
Because test cycles are standardised and gentler than real driving. Short trips, cold starts, high speeds, air conditioning, and heavy loads all reduce economy, typically by 10 to 20 percent.
How do I convert MPG to litres per 100 km?+
Divide 235.2 by the US MPG figure. 30 US MPG is about 7.8 litres per 100 km. Note that imperial gallons are larger, so UK MPG figures use 282.5 instead.
What is the cheapest way to improve fuel economy?+
Correct tyre pressure, removing unnecessary weight and roof racks, and smoother driving with less hard acceleration. Together these commonly yield 10 percent or more at no cost.