Fuel Cost Calculator

What this works out

Given a distance, your vehicle's fuel efficiency and the price of fuel, this returns how much fuel the trip consumes and what it will cost. It is the calculation worth doing before a long drive, when deciding whether to drive or take a train, and when working out what to charge passengers who are sharing the journey.

Efficiency and distance have to be in matching units — kilometres with km/l, miles with mpg. The one trap is L/100km, which runs the other way: it is consumption, not efficiency, so you multiply rather than divide.

A worked example

A 640 km drive in a car returning 16 km/l, with fuel at 105 per litre.

• Fuel needed: 640 / 16 = 40 litres
• Cost: 40 x 105 = 4,200
• Cost per kilometre: 105 / 16 = 6.56
• Split four ways: 1,050 each
• Return trip: 8,400 — an easy thing to forget when comparing against a round-trip train fare

Converting between efficiency units

Vehicles are rated differently depending on where they were sold, and comparing two of them usually means converting first.

The last row catches people out regularly. An imperial gallon is about 20% larger than a US gallon, so the same car is advertised at 40 mpg in the UK and 33 mpg in the US without anything about the vehicle changing.

Why the manufacturer's figure is optimistic

Official economy figures come from standardised laboratory cycles, and real-world consumption is typically 10-25% worse. Rather than trusting the brochure, measure your own: fill the tank completely, reset the trip meter, drive normally until the tank is well down, then fill completely again. Divide the distance travelled by the litres it took to refill. Two or three tanks of this gives a figure far more useful than any published number, and it is the figure to enter here.

What moves it most, roughly in order of impact:

• Speed. Aerodynamic drag rises with the square of speed, so power needed rises with the cube. Most cars are most efficient somewhere around 70-90 km/h; at 120 km/h consumption can be 25-30% higher than at 90.
• Acceleration and braking. Every hard acceleration converts fuel into kinetic energy that the brakes then throw away as heat. Stop-start city traffic is where economy collapses.
• Tyre pressure. Underinflation increases rolling resistance. Running 20% below the recommended pressure costs roughly 2-3% in economy, and wears the tyres faster.
• Load and roof boxes. Weight matters mostly in city driving; a roof box or roof rack matters mostly at motorway speed, where it can cost 10-20%.
• Air conditioning. Worth several percent in traffic. At high speed, running the A/C is usually cheaper than the drag from open windows.
• Cold starts. A cold engine runs rich. Short trips of a few kilometres can consume at twice the steady-state rate, which is why urban economy is so poor.

Fuel cost is not the cost of driving

If you are comparing driving with flying or taking the train, fuel alone understates the case. Tolls, parking and any congestion charge are direct trip costs. Beyond that, distance consumes the vehicle itself: depreciation, tyres, servicing intervals and eventual repairs all scale with kilometres driven. Many organisations reimburse mileage at a rate two to three times the pure fuel cost for exactly this reason, and that reimbursement rate is a more honest figure for the true marginal cost of a journey. For a rough all-in estimate, doubling the fuel figure is not unreasonable for a typical car.

Frequently asked questions