How to Track Fuel Economy Accurately (Fill-to-Fill Method)
June 11, 2026 · 6 min read
Your dashboard’s fuel-economy read-out is an estimate, and usually an optimistic one — real-world tests routinely find trip computers reading 5–10% better than the fuel actually burned. If you want the true number (to spot a developing fault, to verify a “fixed” one, or just to know what your driving costs), there’s exactly one reliable method, and it needs nothing but a fuel log.
The fill-to-fill method
Three steps:
- Fill the tank completely — to the pump’s first automatic click — and record the odometer.
- Drive normally. No need to track anything in between.
- Fill completely again. Record the odometer and the litres the pump shows.
Because both fills go to the same physical level, the second fill’s litres equal the fuel burned over exactly the kilometres between the two odometer readings:
L/100km = litres refilled ÷ km driven × 100
Example: 42.3 L over 612 km → 42.3 ÷ 612 × 100 = 6.9 L/100km. In imperial terms: miles driven ÷ gallons refilled = MPG.
L/100km ↔ MPG, at a glance
| L/100km | MPG (US) | Verdict for a petrol car |
|---|---|---|
| 5.0 | 47 | Excellent — small car or hybrid territory. |
| 7.0 | 34 | Typical for a compact in mixed driving. |
| 9.0 | 26 | Normal for an SUV; high for a compact. |
| 12.0 | 20 | Heavy vehicle, city-only driving — or a fault. |
The conversion is 235.2 ÷ either number (235.2 ÷ L/100km = MPG, and vice versa).
What ruins the measurement
- Partial fills as endpoints. A half-fill tells you nothing about the level in the tank. Log partial fills for cost, but only full-to-full windows count for economy.
- Different pumps, different click points. Pump cut-offs vary slightly; over one tank that’s a ±2–3% wobble. Don’t read meaning into a single tank — trust the average of three or four.
- Topping up past the click. Squeezing in “a bit more” changes the fill level unpredictably (and is bad for the vapour recovery system). Stop at the click, every time.
The trend is the real payoff
One reading tells you little; the trend tells you a lot. Consumption creeping up 10% over a few months is an early-warning light that no dashboard shows: tyre pressures, a lazy oxygen sensor, dragging brakes, or a clogged air filter — all announce themselves at the pump first. Keep the log in a consistent template and the trend is free. If you log your fill-ups in Vroom, the fill-to-fill calculation — including correct handling of partial fills — is done for you, in L/100km or MPG to match your settings.