The usual method, and where it breaks
The classic way to measure fuel economy goes like this: fill the tank to full, note the odometer, drive, fill to full again. The litres you just put in are exactly what you burned since the last fill, so:
L/100 km = litres added ÷ kilometres driven × 100
That works perfectly — as long as every fill is a full one. Real life has partial fills: €20 on the way to work, a splash before a long trip, a cheaper station you only half-fill at. Each partial fill puts in an amount of fuel that has nothing to do with the distance since the previous stop.
What a partial fill does to the numbers
Say you fill to full at 10,000 km. At 10,450 km you put in 10 litres because you’re in a hurry. At 10,700 km you fill to full again and it takes 38 litres.
If you calculate each fill on its own:
| Fill | Litres | Km since last fill | “Economy” |
|---|---|---|---|
| Partial at 10,450 | 10 | 450 | 2.2 L/100 km |
| Full at 10,700 | 38 | 250 | 15.2 L/100 km |
Neither number is real. The first looks like a miracle, the second like a fault. The car did neither.
The fix: measure between two full tanks
Treat every full fill as a checkpoint. Between two checkpoints, add up all the fuel that went in — partial fills included — and divide by the distance between them.
In the example: 10 + 38 = 48 litres over 700 km = 6.9 L/100 km. That is the figure your car actually achieved over those 700 km.
The fill-ups above are a made-up example to show the method, not typical figures for any car.
A few habits keep it accurate:
- Define “full” once and stick to it. Stop at the pump’s first click-off; don’t top up. Consistency matters more than squeezing in the last litre.
- Log every fill, even the small ones. A missed partial fill makes the next window look better than it was. If you do miss one, start counting again from the next full tank.
- Expect noise over short windows. Over a few hundred kilometres, tank shape and pump behaviour still move the number a little. Averages over several windows are the ones to trust.
L/100 km, km/L or MPG?
They all describe the same thing; only the direction differs. L/100 km goes down as economy improves; km/L and MPG go up. To convert:
- km/L = 100 ÷ (L/100 km)
- US MPG ≈ 235.2 ÷ (L/100 km)
- UK MPG ≈ 282.5 ÷ (L/100 km)
Why it is worth the trouble
A reliable economy figure is an early warning. If the same car on the same roads suddenly needs noticeably more fuel, something has changed — tyre pressure, a dragging brake, a sensor, or simply winter. And the distance you log at each fill-up tells you how much you drive, which is exactly what you need to predict when the next service is due.
