Measure your mileage
Fill the tank and note the odometer or reset the trip meter. Drive normally. At the next fill-up, note the miles and the gallons it takes to fill the tank again. Divide miles by gallons. That is your miles per gallon.
For example, 410 miles on 13.6 gallons is 30.15 miles per gallon, or 3.32 gallons per 100 miles. At $3.40 a gallon that is $46.24, about 11 cents a mile. Averaging several fill-ups smooths out differences in how full the pump left the tank.
Estimate a trip
Gallons needed equal miles divided by miles per gallon. A 180-mile one-way trip at 26 miles per gallon uses about 6.92 gallons each way. Round trip is 360 miles and 13.85 gallons, so $47.08 at $3.40 a gallon.
Highway speed, headwinds, hills, cold weather, a loaded roof rack and under-inflated tires can all lower real-world mileage. The U.S. Department of Energy and EPA publish fuel economy information at FuelEconomy.gov.
Business and tax note
If you drive for business, a fuel-cost estimate is not the same as a deduction. The IRS publishes standard mileage rates each year, and records of your trips matter. Check the current IRS guidance or ask a tax professional.
Try the calculator
Run your own numbers in the free gas mileage calculator. It works in your browser and nothing you enter is saved or sent.
Sources
Common questions
Why is my mileage lower than the sticker?
The sticker is a standardized test. Driving style, conditions and vehicle condition change the result.
Is the dashboard number accurate?
It can be off by a little. Full tank to full tank is usually closer.
Does the estimate include tolls?
No, only fuel.
This guide is general information, not financial, tax or legal advice. Rates, fees and rules change, so confirm current terms with the lender, store or agency involved. All dollar figures are invented examples.