Road Trip Fuel Cost Calculator
Distance, consumption and pump price in whichever units your car and your country use — with the return leg, the split between passengers and a comparison against another car or an EV.
Nothing you type here is uploaded, stored or sent anywhere. It all runs in your browser.
The conversions people get wrong
A US gallon is 3.785 litres. An imperial gallon is 4.546. That is a 20% difference, and it means a car quoted at 40 MPG in Britain is about 33 MPG in America — the same car, the same fuel, two numbers. Any calculator that treats MPG as one unit is out by a fifth for half its users, and it will never tell you which half you are in.
The other trap is direction. Litres per 100 km goes up as the car gets worse; MPG and km per litre go down. They are reciprocals, so you cannot average them and you cannot interpolate between them. Two cars at 30 and 60 MPG driven the same distance do not average 45 MPG — they average 40, because the thirsty one burns more of the fuel.
For the record, the conversions this page uses: litres per 100 km = 235.21 ÷ US MPG, = 282.48 ÷ imperial MPG, = 100 ÷ km per litre. A mile is 1.609344 km, exactly.
Why your real cost is higher than this
- Official consumption figures are optimistic. Manufacturer numbers come from a standardised test cycle, not from your motorway at 120 km/h with four people and a roof box. Use the figure from your own trip computer, or add 10 to 20% to the brochure.
- Speed costs fuel faster than it saves time. Drag rises with the square of speed, so the power needed rises with the cube. Going from 110 to 130 km/h typically costs 15 to 25% more fuel to save about nine minutes per hundred kilometres.
- A roof box is expensive. Ten to twenty per cent at motorway speed, for the whole trip, not just the part where you needed it.
- Mountains are not symmetrical. What you spend climbing a pass you do not get back on the descent. A route with 2,000 m of climbing costs meaningfully more than its distance suggests, which is the one way a road trip resembles a hike.
- Fuel is not the whole cost of driving. Tolls, vignettes, parking, tyres and wear are real money. Fuel is simply the part that is easy to calculate, and comparing a drive against a flight on fuel alone flatters the drive.
Comparing against an electric car honestly
The comparison is only as good as the electricity price you put in, and that price varies more than petrol ever does. Charging at home overnight and charging at a motorway rapid charger can differ by a factor of three or four, which is the difference between an EV road trip costing a third of a petrol one and costing roughly the same. If you are comparing a long drive, use the rapid-charging price, because that is what a long drive actually uses. Consumption also climbs at motorway speed — more sharply than a combustion car's does, because there is no gearbox to hide behind.
Questions people ask
Is MPG the same in the US and the UK?
No. The gallons are different sizes: 3.785 litres in the US, 4.546 imperial. The same car reads about 20% better in imperial MPG. Pick the right option on this page or the answer will be wrong by that much.
What consumption figure should I use?
The long-term average from your own trip computer, if you have one. Failing that, take the manufacturer’s combined figure and add 10 to 20% for motorway driving with a full car. Official figures come from a test cycle that is gentler than any real trip.
Does this include tolls and parking?
No — it costs the fuel only. On some routes tolls and vignettes are a third of the total cost of driving, so look them up separately before deciding the drive is cheaper than the train.
How should I split the cost between passengers?
This page divides it evenly, which is the usual convention and is what the driver almost never asks for. If you want to be precise about it, the driver is already contributing the car, its wear and the insurance, so an even split of fuel is generous to the passengers rather than to the driver.
Are the figures I enter stored?
No. The calculation runs in your browser, there is no backend on this page, and nothing is uploaded or saved.
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