Fuel Cost Calculator: How It Works
Fuel cost for a journey is distance divided by efficiency, multiplied by price. The arithmetic is trivial; the useful part is knowing which of your own habits move the result, because several of them move it by 20% or more.
The formula
Cost = (Distance ÷ Efficiency) × Fuel price
A 450 km trip in a car doing 14 km per litre, with fuel at 1.65 per litre: (450 ÷ 14) × 1.65 = 32.1 litres × 1.65 = 53.00.
Efficiency units
| Unit | Used in | Higher is |
|---|---|---|
| km per litre | India, Japan, much of Asia | Better |
| litres per 100 km | Europe, Australia, Canada | Worse |
| miles per gallon (US) | United States | Better |
| miles per gallon (imperial) | United Kingdom | Better |
The two gallons are different: an imperial gallon is about 4.55 litres and a US gallon about 3.79. A car rated at 40 mpg in the UK is roughly 33 mpg by US measurement, for identical real efficiency. Comparing figures across the Atlantic without converting overstates British cars by about 20%.
Litres per 100 km inverts the relationship — lower is better — which is why it is the more useful unit for comparing running costs directly.
What actually changes your consumption
| Factor | Typical effect |
|---|---|
| Speed above roughly 90 km/h | Aerodynamic drag rises with the square of speed; 120 km/h can use 20–30% more than 90 |
| Aggressive acceleration and braking | 10–30% worse in town |
| Under-inflated tyres | Up to 3% worse |
| Roof box or bike rack | 10–25% worse at motorway speed, even empty |
| Air conditioning | 5–20% worse, most noticeably in town |
| Extra weight | Around 1–2% per 45 kg |
| Short cold trips | Substantially worse until the engine warms |
Speed and the roof box are the two large ones, and the roof box is the one people forget: an empty roof rack left on for months costs real money on every motorway journey.
Why you never get the official figure
Manufacturer economy figures come from standardised laboratory cycles. Real-world consumption is typically 10–25% worse, because the tests use gentle acceleration, moderate speeds, no auxiliary loads and ideal conditions. Calculate with your own measured figure — fill the tank, note the odometer, drive normally, refill and divide — rather than the brochure number.
Comparing options for a journey
Fuel is only part of a car's cost. A fuller comparison for a long trip includes tolls, parking and a share of maintenance and depreciation, which many organisations approximate with a per-kilometre rate rather than fuel alone. Against a train or coach fare, comparing fuel alone systematically flatters driving.
Sharing the cost
Divide the total by the number of people, including the driver — the driver is travelling anyway, and charging passengers the full cost turns a shared journey into a taxi fare. Where the driver bears wear and tear, a modest uplift is reasonable, agreed before departure rather than at the destination.