Start with two complete fill-ups
The simplest tank-to-tank calculation needs a starting odometer reading and the gallons added at the next complete fill-up. Divide miles driven by gallons added. The first entry establishes a starting point; it cannot calculate fuel economy by itself.
Be consistent at the pump
Automatic shutoff can vary between pumps, vehicle angles, and fuel flow rates. Avoid repeatedly topping off after the nozzle stops. A small difference can distort one result, especially on vehicles with small tanks. Over several tanks, ordinary variation usually becomes less important.
Judge the trend, not a single number
Weather, idling, short trips, towing, tire pressure, fuel blend, traffic, and driving speed can all change consumption. Compare several tanks under similar use. A sustained drop deserves attention; one unusual tank usually deserves a note.
Check the odometer entry
A misplaced digit can produce an impossible result. Record the full odometer value rather than trip mileage, and make sure the new reading is greater than the previous one. If you miss a fill-up, do not force a tank calculation; wait until the next reliable pair of records.
Use fuel data as a clue
Fuel economy can help identify a change, but it cannot diagnose the cause by itself. If a lasting decrease appears alongside warning lights, rough running, fuel odor, overheating, or drivability problems, arrange appropriate inspection rather than relying on the number alone.