Fuel cost and MPG calculator
A 1,200 km drive at 6.5 l/100 km burns 78 litres, which is two refuels on a 45 litre usable tank. Each stop is realistically twenty minutes once you have queued, paid and used the facilities, and it is where the coffee and sandwich money goes. The fuel figure is exact; the trip budget is that plus about the same again in stops if there are four of you.
Fuel cost is distance divided by economy, multiplied by the pump price of petrol or diesel: 300 miles at 45 imperial MPG is 6.67 gallons, about £44 at £1.45 a litre. The same panel measures what the car really returns, so 300 miles on 10 US gallons is 30 MPG, or 7.84 litres per 100 km.
How to work out fuel cost
Fuel is rarely the whole cost of driving, and treating it as such makes driving look cheaper than it is. Tyres, servicing, depreciation and the share of insurance and tax attributable to the mileage roughly double the real per-mile figure for most cars. Government mileage allowances exist precisely because of this: the UK rate of 45p a mile and the US IRS rate are both several times the pure fuel cost. If you are working out what to charge someone, the fuel figure here is the floor, not the answer.
The two gallons are the reason the economy modes print everything at once. A US gallon is 3.785 litres and an imperial gallon is 4.546, so the same car returns about 20% more MPG on paper in Britain than in America without burning a drop less fuel. Comparing a US and a UK figure directly is one of the most common apples-to-oranges errors in car buying, and it always flatters the British number.
MPG also has an awkward property: it is inversely proportional to fuel used, so equal improvements are not equal savings. Going from 15 to 20 MPG saves more fuel over a given distance than going from 40 to 60. Litres per 100 km avoids the whole problem because it is linear in cost — 8 to 6 l/100 km is exactly a quarter less fuel — which is why most of the world labels consumption that way, and why both figures are shown side by side here.
For a road trip, take your real consumption rather than the brochure figure and add about ten per cent. Sustained motorway speed costs more than the mixed cycle the official test uses, a roof box adds 10 to 20% because drag rises with the square of speed, and four people and their luggage add a few per cent more. Tolls are not included anywhere here, and on some routes they rival the fuel.
What people use it for
- Splitting fuel fairly on a shared drive
- Deciding between driving and taking the train
- Estimating a delivery or commute cost
- Comparing two routes with different distances
- Measuring real economy brim to brim after a fill-up
- Converting between US MPG, imperial MPG and litres per 100 km
- Budgeting a road trip including its refuel stops
- Checking real economy against the manufacturer figure
- Spotting a developing fault from a drop in economy
- Planning a multi-day drive across several countries
- Working out a cost per mile to put on an expense claim
- Averaging economy over several tanks rather than trusting one fill
- Comparing two cars whose figures are quoted in different systems
Questions
Divide the price per gallon by the MPG. At £6.59 a gallon and 45 MPG that is about 15p a mile in fuel alone.
Divide the miles driven since the last fill by the gallons it took to refill. Measure over a full tank for a figure worth trusting.
Yes, it is the American name for it. The catch is the gallon: a US gallon is smaller, so a car that reads 45 MPG in Britain reads about 37 in the same conditions in America.
An imperial gallon is 4.546 litres against a US gallon of 3.785, so the same car scores about 20% higher in imperial. UK and Irish figures are imperial; US and Canadian ones are not.
Divide 235.2 by the US MPG figure, or 282.5 by the imperial one. The tool prints both alongside every result.
For money, yes, because it is linear: 8 to 6 l/100 km saves exactly a quarter of the fuel, whereas the same MPG improvement saves less the higher the number already is.
Most mileage allowances think so. They cover wear, tyres and depreciation as well, and run at roughly double the fuel-only figure. An equal split of fuel alone is fair between people who share the driving and the car; where one person provides the car, most groups add something on top rather than splitting the fuel four ways.
For one person, often the train. For three or four sharing a car, driving usually wins, which is what the per-person row is for.
Usually yes above about 55 mph, where aerodynamic drag dominates. Dropping from 80 to 70 mph typically saves around 15%.
Yes: commonly 10 to 20% at motorway speed. Take it off for the legs of the trip where you do not need it.
Divide the trip fuel by your usable tank, which is typically the tank size less a 10% reserve. The refuel-stop row does it for you.
Distance divided by economy times price, using your real-world consumption plus about 10% for motorway speed and a loaded car.
Only if you are making it. The tool shows the one-way and there-and-back figures so there is no ambiguity.
This tool covers fuel only. Tolls on some routes rival the fuel cost, so look them up separately.
Short cold trips, motorway speed, tyre pressure, roof bars and a full boot all move it. Cold-start running is the single largest factor for urban driving.
It depends entirely on the vehicle. What matters more is your own trend: a sudden 10% drop usually means tyre pressure, a sensor or a dragging brake.
Rarely exactly. Most trip computers read a few per cent optimistic. Brim-to-brim is the honest figure.
Three is enough to smooth out fill-level error and one unusual week of driving.