Your car's real fuel economy is easy to measure with two numbers from each fill-up: the miles you drove since the last fill-up and the gallons it took to fill the tank again. Averaging several tanks smooths out the differences between a highway week and a city week.
How to measure your MPG
- Fill the tank until the pump clicks off the first time. Don't top off.
- Reset the trip meter (or note the odometer).
- Drive normally until you need fuel again.
- Fill up the same way and note the gallons on the receipt and the miles on the trip meter.
- Divide miles by gallons. 312 miles ÷ 10.4 gallons = 30.0 mpg.
Add each fill-up as a row above. The average is total miles ÷ total gallons, which weights longer tanks correctly, rather than an average of the individual MPG figures.
Comparing with the EPA rating
Pick your car to see how you compare with its official EPA combined rating. A result within roughly 10% either side is common. Bigger gaps usually have ordinary causes:
- Driving mix: lots of short trips, idling or high highway speeds.
- Weather: cold temperatures and winter fuel blends reduce MPG; so does running the A/C hard.
- Load and aerodynamics: roof racks, cargo boxes, towing.
- Maintenance: under-inflated tires, a clogged engine air filter or a dragging brake.
- Measurement: a single tank can be off if the pump clicked off early or late.
A sudden drop that you can't explain is worth a look. A check engine light with a lean or misfire code often comes with worse fuel economy.
Converting units
- L/100 km = 235.215 ÷ MPG (US gallons). 30 mpg ≈ 7.8 L/100 km.
- km/L = MPG × 0.4251.
Frequently asked questions
How do you calculate miles per gallon?
Divide the miles driven between fill-ups by the gallons needed to refill the tank. Filling to the first click both times makes it accurate.
How many fill-ups do I need for an accurate average?
Three or more. One tank can be skewed by where the pump clicked off or by an unusual week of driving.
Why is my real MPG lower than the EPA rating?
EPA ratings come from standardized lab tests. Short trips, cold weather, high speeds, heavy loads and tire pressure all move real-world results. Some drivers beat the rating on gentle highway driving.
Can CarManager track this automatically?
A free CarManager garage keeps a fuel log for each car, works out your MPG and cost per mile, and compares it with the EPA rating.
Results are estimates from the numbers you enter and, where noted, EPA ratings. Not financial advice. Page updated .