Auto · 5 min read · Last updated September 2026
Worked example: 15,000 km at 15 km/L needs 1,000 L; at 100 per litre that is 100,000 for the year. In US units: 12,000 miles at 25 mpg needs 480 gallons; at $3.50/gal that is $1,680.
| Efficiency | Fuel for 15,000 km | Cost @ 100/L |
|---|---|---|
| 12 km/L | 1,250 L | 125,000 |
| 15 km/L | 1,000 L | 100,000 |
| 18 km/L | 833 L | 83,333 |
The jump from 12 to 15 saves 25,000 a year; 15 to 18 saves 16,667. Efficiency improvements have diminishing returns — which is why the biggest lever is usually total distance driven, not the last km/L.
Manufacturer figures are lab-cycle numbers; city driving, short trips, roof racks and cold starts commonly cost 10–25% vs the brochure. Measure your own consumption over two or three full tanks (fill to the same point, note litres and km), then use your figure in the Fuel Cost Calculator — and the efficiency converter if you need to move between km/L, mpg and L/100km.
Distance ÷ efficiency × fuel price. A 500 km trip at 15 km/L with fuel at 100/L: 500 ÷ 15 = 33.3 L × 100 = 3,333.
At 15 km/L and 100 per litre: 15,000 ÷ 15 × 100 = 100,000 per year. Substitute your own efficiency and fuel price for your real number.
L/100km is linear: halving consumption halves the figure, which makes savings easier to reason about. km/L grows without bound. Either works if you run the formula consistently.
Measured consumption is usually worse than brochure figures: idling, short trips, traffic and load all reduce efficiency. Meter your own tanks and use that number.