A baseline, not a promise
Standardized ratings make different vehicles easier to compare. A personal commute has its own speeds, temperatures, hills, stops and loads. Vehicle condition and driving behavior also influence consumption. Your average can therefore differ from the EPA rating without making either number meaningless.
Measure across more than one fill-up
For a gasoline vehicle, record miles driven and gallons added over several full-to-full refueling intervals. Divide total miles by total gallons. Be consistent about how you fill the tank; one short interval is more sensitive to pump shutoff differences. Do not average individual tank MPG figures when the tank distances differ—use total distance divided by total fuel.
A small MPG drop can change the budget
At 12,000 miles and $3.50 per gallon, a car returning 30 MPG costs $1,400 per year to fuel. If your measured average is 25 MPG, the estimate rises to $1,680. That $280 difference comes from consumption alone, without any change in price or annual distance.
Keep the calculation honest
Use recent measured consumption for a personal estimate and official ratings for standardized comparisons. When planning unfamiliar conditions, test more than one consumption value. We do not apply a universal winter or highway penalty because the effect depends on the specific car and journey.