⛽ Fuel Cost Calculator
Work out what a journey costs in fuel from the distance, your vehicle's economy and the price at the pump. It handles MPG in both US and imperial gallons, litres per 100 km and kilometres per litre.
What a journey costs, and the economy figures that hide the answer
Fuel cost is distance divided by economy times price. The arithmetic takes five seconds; the units take longer, because the world measures fuel economy in at least four incompatible ways, two of which are called MPG.
cost = (distance / economy) × price per unit of fuel
Four ways to say the same thing
| Measure | Used in | Higher is |
|---|---|---|
| MPG, US gallons | United States | Better |
| MPG, imperial gallons | United Kingdom | Better |
| Litres per 100 km | Most of Europe, Australia | Worse |
| Kilometres per litre | Japan, India, Latin America | Better |
An imperial gallon is 20% larger than a US one, so the same car rated at 30 MPG in Britain is only 25 MPG in America. Nothing changed except the gallon. And litres per 100 km runs in the opposite direction to the other three, which makes casual comparison across sources genuinely error-prone.
MPG (US) = 235.21 / (L/100 km) MPG (imperial) = 282.48 / (L/100 km)
Why L/100 km is the better measure
MPG is deceptive because it is a reciprocal. Improving from 10 to 15 MPG saves far more fuel over the same distance than improving from 30 to 35, even though both are a five-unit gain. Over 10,000 miles: the first saves 333 gallons, the second saves 48. Litres per 100 km is linear in fuel consumed, so the numbers mean what they appear to mean, which is why regulators in most of the world prefer it.
This is sometimes called the MPG illusion. If you own two cars, replacing the thirsty one with something slightly better saves more fuel than replacing the efficient one with something excellent.
What ruins real-world economy
| Factor | Effect on consumption |
|---|---|
| Motorway speed 110 km/h vs 90 km/h | +15 to 25% |
| Roof box or roof bars fitted | +10 to 20% at motorway speed |
| Tyres 20% underinflated | +3 to 5% |
| Short urban trips from cold | +20 to 40% |
| Air conditioning in heavy traffic | +5 to 10% |
| 100 kg of extra load | +1 to 2% |
Published figures come from standardised test cycles that no real journey resembles. Even the newer WLTP cycle in Europe, which is more realistic than the one it replaced, typically runs 10% to 20% better than drivers actually achieve. Use your own measured economy if you have it: fill the tank, reset the trip meter, and divide at the next fill.
Cost per mile and the rest of the bill
Fuel is the visible cost and rarely the largest one. Depreciation on a newer car commonly exceeds fuel by a factor of two or three, and insurance, maintenance and tyres add more. A useful split: fuel is the marginal cost of a journey you have already decided to make, while the full cost per mile is what matters when deciding whether to own the car at all.
A worked example
A 450-mile trip in a car doing 32 MPG US, with fuel at $3.45 a gallon. Fuel used: 450 / 32 = 14.06 gallons, costing $48.52, which is 10.8 cents a mile. Shared between four people that is $12.13 each, comfortably less than one rail ticket. Fit a roof box and economy drops to around 27 MPG, pushing the cost to $57.50.
Frequently Asked Questions (FAQs)
How do I convert MPG to litres per 100 km?
Divide 235.21 by the US MPG figure, or 282.48 by the imperial one. A car doing 32 MPG US uses 7.35 L/100 km. The conversion is a division rather than a multiplication because the two measures run in opposite directions.
Why is UK MPG higher than US MPG for the same car?
An imperial gallon is 4.546 litres against 3.785 for a US gallon, 20% larger. The car travels the same distance on the same fuel; only the size of the gallon changes, which inflates the British figure by about a fifth.
Does driving slower really save much fuel?
Yes, above about 80 km/h. Aerodynamic drag rises with the square of speed, so the power needed rises with the cube. Dropping from 110 to 90 km/h typically cuts consumption by 15% to 25%, at a cost of a few minutes on a typical journey.
How accurate are manufacturer economy figures?
Optimistic. Official test cycles are standardised rather than realistic, and real-world consumption typically runs 10% to 20% worse even under the newer WLTP protocol. Measuring your own over several tanks gives a far better number.
Should I use the trip computer's figure?
It is a reasonable guide and usually optimistic by 2% to 5%. For an accurate figure, fill the tank to the same point twice, note the distance between fills and divide by the litres added. Repeat over three tanks to smooth out filling differences.
Is it cheaper to drive or take the train?
On fuel alone, driving usually wins with two or more passengers. On the full cost per mile including depreciation, insurance and maintenance, the train is often competitive for one person on a long trip. Compare fuel against the ticket for a journey you are making anyway, and full cost against it when deciding whether to own a car.