Fuel Cost & Mileage Calculator
Trip cost, fuel economy in every common unit, and cost per kilometre.
Last updated
Your details
Result
- Fuel economy
- 12.50 km/L
- Litres per 100 km
- 8.00
- Miles per gallon (US)
- 29.40
- Miles per gallon (UK)
- 35.31
- Cost of this trip
- $4,000.00
- Cost per km
- $8.00
The European convention — lower is better
Three ways to express the same thing
Kilometres per litre and miles per gallon both rise as efficiency improves. Litres per 100 km runs the other way — lower is better — because it measures consumption rather than distance.
The inverted scale matters when comparing improvements. Going from 5 to 4 L/100km saves far more fuel over a year than going from 10 to 9 L/100km, even though both look like a one-unit gain.
Measuring your real economy
Fill the tank completely, reset the trip meter, drive normally for at least a few hundred kilometres, then fill completely again. The litres it takes to refill divided into the distance is your true economy.
A single tank is noisy — traffic, load and weather all move it. Averaging three or four tanks gives a figure worth comparing against the manufacturer's claim.
What actually improves economy
Speed is the largest controllable factor. Aerodynamic drag rises with the square of velocity, so fuel consumption climbs sharply above roughly 90 km/h. Reducing motorway speed by 10 km/h commonly improves economy by 10% or more.
Tyre pressure matters more than most people expect: underinflation by 20% can cost several percent in fuel. Removing roof boxes and unnecessary weight, and avoiding hard acceleration, each contribute a few percent more.
Comparing running costs honestly
Cost per kilometre is the figure that allows a fair comparison between vehicles and between fuels. It folds together economy and fuel price, which is why a more efficient car running on expensive fuel can cost the same to run as a thirstier one on cheap fuel.
For a complete picture, add depreciation, insurance, servicing and tyres. Fuel is the most visible running cost but frequently not the largest over a vehicle's life.
Formula & methodology
What this calculator assumes
- A US gallon is 3.785411784 litres and an imperial gallon 4.54609 litres, so the same car rates about 20% higher in imperial mpg than in US mpg. Comparing an American figure with a British one without converting is the usual mistake.
- Litres per 100 km is an inverse measure: lower is better, and it does not scale linearly against mpg.
- Uses the economy you enter. Real consumption varies with load, terrain, traffic, tyre pressure and driving style, and manufacturer figures are measured on a test cycle rather than on a road.
Sources
Frequently asked questions
Why is my real economy worse than the official rating?
Official figures come from standardised laboratory cycles with no wind, cargo, air conditioning or aggressive acceleration. Real-world consumption is commonly 15–25% higher.
Does air conditioning use much fuel?
Around 5–10% in city driving. At motorway speeds it is usually cheaper than opening the windows, which increases aerodynamic drag.
Is it worth driving further for cheaper fuel?
Rarely. Saving a small amount per litre on a 50-litre fill is quickly eaten by a detour, especially once time is valued at all.