Your car knows its MPG — it displays it confidently every trip. It's also usually wrong, in the flattering direction. Here's the measurement method that costs nothing and never lies.
The method: fill-up to fill-up
- Fill the tank completely — same nozzle click-stop every time. Reset your trip odometer (Trip A).
- Drive normally until the next fill-up. No coasting downhill with the engine off to look good.
- At the next fill-up: note trip miles and gallons pumped. MPG = miles ÷ gallons. Reset the trip meter and repeat.
That's it. The pump and odometer are both calibrated instruments; the arithmetic between them is exact. Our gas mileage calculator does the division and converts it to cost per mile and per 100 miles at your local price.
Why the dashboard overstates
Cars estimate economy from injector pulse and airflow models, and independent tests routinely find dashboards optimistic by ~5–10% — some vehicles by double digits. The bias isn't a conspiracy, just calibrated-for-flattery engineering. The fill-up method has no such incentive.
Getting an average you can trust
One tank is an anecdote: a highway-heavy tank can read 15% above a city tank in the same car. Track 3–4 consecutive tanks and average them (total miles ÷ total gallons — the weighted way). Track it seasonally if you're curious: winter blends and cold starts reliably knock 10–20% off summer numbers in northern climates.
What your number is telling you
- Within 10% of EPA combined: normal. EPA lab cycles are gentle; real driving isn't.
- 20%+ below: check the cheap stuff first — tire pressure (underinflation alone can cost 1–2%), a dragging brake, an old air filter, or aggressive short-trip patterns where the engine never fully warms.
- Suddenly worse: that's diagnostic information. Gradual drift is maintenance; a sudden drop is a fault code waiting to happen.
The budget step everyone skips
MPG alone doesn't budget. Convert it: cost per 100 miles = gas price ÷ MPG × 100. At $3.45 and 28 MPG that's $12.32 per 100 miles — multiply by your monthly miles and you have the fuel line of your budget, exact to the dollar. It also makes car comparisons honest: 28 vs 33 MPG sounds minor until the per-100 figure shows $1.05 difference — $315 a year at 30,000 miles.