GarageCalc 
The fill-up method, step by step
- Fill the tank until the pump clicks off. Reset the trip odometer.
- Drive normally until the tank is at least half empty (longer stretch = more accurate).
- Fill up at the same pump speed until it clicks off again.
- Enter the trip miles and the gallons from the receipt above.
Combine several fill-ups by dividing total miles by total gallons to reduce the influence of a single fill-level difference. For trip planning with that estimate, use the fuel cost calculator.
If you are comparing that result with a window sticker, read what combined MPG means before assuming the EPA figure describes your driving mix.
How it's calculated
Changed tire size? Your MPG math is skewed
The odometer counts wheel revolutions calibrated to the factory tire. Taller tires make it under-count miles, so your calculated MPG reads worse than reality; shorter tires do the opposite, if the vehicle has not been recalibrated. Estimate the distance correction with the tire size comparison calculator before dividing. The bigger tires and speedometer guide explains the ratio and why the percentage depends on the baseline.
Worked fill-up: 350 miles and 12.5 US gallons
Divide 350 by 12.5 to get 28 US MPG. The metric equivalent is 235.215 ÷ 28, or about 8.4 L/100 km. At $3.20 per gallon, that fill-up costs $40 and fuel costs about 11.4 cents per mile. This excludes maintenance, insurance and other ownership costs.
Use the fuel added at the end of the interval, not the gallons from the receipt when you reset the trip meter. Both endpoints need comparable full-tank levels. Stop at the first automatic shutoff rather than topping off, and keep the receipts with their odometer readings.
How to average MPG over multiple tanks
Suppose one tank covers 300 miles on 10 gallons (30 MPG), and the next covers 200 miles on 8 gallons (25 MPG). The combined result is 500 ÷ 18 = 27.8 MPG, not the simple average of 30 and 25, which is 27.5. Total distance divided by total fuel gives each tank the correct weight.
These examples use US gallons. An imperial gallon is larger, so UK MPG cannot be compared directly with US MPG. The calculator displays both. For another fill-up calculation and US vehicle comparison data, see the EPA/DOE fuel economy calculator.
Frequently asked questions
How do I calculate my real MPG?
Fill to the first automatic pump shutoff and reset the trip odometer or note the mileage. At the next comparable fill-up, divide miles driven by US gallons added. This estimates fuel economy for that interval. Consistent fill levels and accurate distance readings matter.
Why does my dashboard MPG differ from the calculated one?
A trip computer estimates fuel use, while this method uses pump volume. Different reset intervals and fill levels can produce different results. Tire-size changes can also affect recorded distance if the vehicle has not been recalibrated. Compare several matching intervals rather than assuming a universal dashboard error.
What is a good MPG?
Compare with the EPA rating for your exact model, engine and drivetrain, using the city or highway figure that best matches your driving. Temperature, speed, short trips, load and traffic affect actual results. A larger vehicle and a compact hybrid are not useful direct comparisons.
How do I convert MPG to liters per 100 km?
Divide 235.215 by the MPG figure. For example, 28 MPG equals 8.4 L/100km. This calculator shows both automatically, plus imperial MPG used in the UK.