How to Calculate Gas Mileage (MPG) the Accurate Way
By Eyal Malka, Founder of Milebook
November 9, 2026 · 5 min read · Fact-checked against 1 primary source
Your car's window sticker gives you an estimate. The trip computer gives you another. Neither one is the number you actually get with your driving, your roads and your fuel. Working out your real miles per gallon (MPG) takes two fill-ups and one division.
The full-tank method, step by step
- Fill the tank until the pump clicks off. Don't top off.
- Write down the odometer reading, or reset a trip meter to zero.
- Drive normally until you need fuel again.
- Fill up all the way again, and note the gallons from the pump and the new odometer reading.
- Divide the miles driven by the gallons you just added.
This is the same method the U.S. government's FuelEconomy.gov site describes for checking your own MPG. It works because the second fill-up replaces exactly the fuel you burned since the first one.
A worked example
| Fill-up | Odometer | Gallons added |
|---|---|---|
| First (full tank) | 42,180 | Starting point |
| Second (full tank) | 42,492 | 11.6 |
Miles driven: 42,492 − 42,180 = 312 miles. MPG: 312 ÷ 11.6 = 26.9 MPG.
What if you didn't fill the tank all the way?
A partial fill-up breaks the simple version, because you can't tell how much fuel is still in the tank. The fix: keep logging, and calculate across the whole stretch from one full tank to the next full tank.
- Add up the gallons from every fill-up after the starting full tank, partial ones included, up to and including the next full tank.
- Divide the total miles over that stretch by that total.
For example: full tank at 10,000 miles, a partial 5 gallons at 10,150, then a full 7.2 gallons at 10,330. That's 330 miles on 12.2 gallons, or 27.0 MPG.
Why your number differs from the dashboard
Trip computers estimate fuel use from injector timing and sensor data. They're usually close, but it's common for them to read a little optimistic. The pump is the measurement you pay for, so the full-tank method is the one to trust when you're comparing tanks or budgeting.
Single tanks also bounce around. Winter fuel blends, a long highway trip, an idling week in traffic or a roof rack can all move one tank a few MPG. Look at the average over several tanks before you worry.
When a drop in MPG means something
A sudden drop that lasts more than a tank or two, with no change in how or where you drive, is worth a look. Common causes:
- Low tire pressure
- A dragging brake or a stuck caliper
- A failing oxygen sensor or thermostat (often with a check-engine light)
- An overdue air filter or spark plugs on older cars
See our guide on how to improve gas mileage for the habits that make the biggest difference.
Turn MPG into cost per mile
Fuel cost per mile = price per gallon ÷ MPG. At $3.50 a gallon and 26.9 MPG, fuel costs about 13 cents a mile. Fuel is only part of what a car costs, though. Here's how to work out your full cost per mile.
Let an app do the math
Milebook's fuel log uses the full-tank method automatically, including partial fills. Log the odometer, gallons and price at each fill-up and tick whether you filled the tank. It keeps a running MPG per tank and an average, charts the trend, and flags a tank that comes in well below your normal.
Track it without a spreadsheet
Milebook logs fuel, expenses, service and trips for every vehicle. Free for 2 vehicles, no card.
Track your MPG free
About the author
Eyal Malka, Founder of Milebook
Eyal Malka is the founder of Milebook. For more than a decade he has run a home-service business in New York City, with work vans and trucks on the road every day. He tracks fuel, repairs and business mileage for those vehicles himself and claims the mileage deduction every year. Milebook started as the tool he wanted for his own vehicles: one place to see what each one really costs, and a mileage log that's ready at tax time.