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CalculatorFuel Cost Calculator
Enter a trip distance, your vehicle's efficiency in whichever unit your dashboard shows, and the fuel price, to get the fuel needed and what the trip costs.
Trip cost
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Efficiency is a reciprocal, and that changes how "percent improved" behaves
Fuel used is distance divided by efficiency — fuel = distance ÷ (km per litre). Because efficiency sits on the bottom of that fraction, a fixed percentage improvement in efficiency always produces the same percentage cut in fuel used, no matter how efficient the vehicle already was. A 25% efficiency gain always means paying for 20% less fuel (since 1 ÷ 1.25 = 0.8), whether you are going from 8 to 10 km/L or from 20 to 25 km/L.
What is not the same is the absolute amount of fuel that 20% represents, because a thirstier vehicle was using more fuel in the first place. Twenty percent of a big number is a bigger number than twenty percent of a small one.
| Efficiency change | Fuel before | Fuel after | Litres saved | % fuel cut |
|---|---|---|---|---|
| 5 → 6.25 km/L | 20.00 L | 16.00 L | 4.00 L | 20% |
| 8 → 10 km/L | 12.50 L | 10.00 L | 2.50 L | 20% |
| 10 → 12.5 km/L | 10.00 L | 8.00 L | 2.00 L | 20% |
| 15 → 18.75 km/L | 6.67 L | 5.33 L | 1.33 L | 20% |
| 20 → 25 km/L | 5.00 L | 4.00 L | 1.00 L | 20% |
| 30 → 37.5 km/L | 3.33 L | 2.67 L | 0.67 L | 20% |
Every row cuts fuel by exactly a fifth. But the top row, an old thirsty engine going from 5 to 6.25 km/L, saves six times more fuel over the same 100 km than the bottom row's already-efficient car improving by the same 25%. This is the real-world reason that fleet efficiency rules focus effort on the worst vehicles first: upgrading a fuel-guzzler by a given percentage recovers far more actual fuel than the same percentage gain on something already efficient.
Why "MPG" needs a country attached to it
A US gallon is 3.785411784 litres; a UK (imperial) gallon is 4.54609 litres — about 20% larger. The same fuel-economy number quoted as "30 MPG" means noticeably different real efficiency depending on which gallon is meant, which is why this calculator asks explicitly rather than assuming one.
Questions people ask
Does this account for traffic, terrain or driving style?
No — it applies your entered efficiency figure uniformly across the whole distance. Real-world consumption varies with hills, traffic and load, so treat the result as a planning estimate based on the efficiency figure you provide, not a guarantee.
Where do I find my vehicle's efficiency figure?
Manufacturer specifications, the trip computer built into many vehicles, or your own calculation from a recent fill-up: distance travelled divided by fuel used since the last full tank.
Can I use this for a round trip?
Yes — enter the total distance for both directions, or calculate one way and double the resulting cost.