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How to Calculate the Fuel Cost of a Trip
Fuel cost is distance divided by efficiency times price. The formula is easy; using your car's real consumption rather than its advertised figure is what makes the answer accurate.
Fuel cost for a journey is distance divided by fuel efficiency, multiplied by the price per unit of fuel. The arithmetic takes seconds. The accuracy depends entirely on using a consumption figure that reflects how your vehicle actually performs rather than the number on the manufacturer's sheet.
The formula in both unit systems
The metric version uses litres per 100km, where a lower number is better. The imperial version uses miles per gallon, where a higher number is better. The two run in opposite directions, which is worth remembering when reading specifications from different markets.
Metric: Cost = (Distance in km / 100) x (Litres per 100 km) x Price per litre Imperial: Cost = (Distance in miles / Miles per gallon) x Price per gallon
Worked example: a long drive
A 640km trip. The car averages 7.4 litres per 100km. Fuel costs 1.68 per litre.
(640 / 100) x 7.4 = 47.36 litres. At 1.68 per litre, that is 79.56.
In imperial: a 400-mile trip at 38 mpg with fuel at 6.40 a gallon gives 400 / 38 = 10.53 gallons, or 67.37.
For a return journey, double it — and if the return leg is loaded differently, with passengers or luggage, expect slightly higher consumption on that leg.
Finding your real consumption
Real-world consumption commonly runs 10% to 25% above the official figure, and the gap is larger for short urban journeys and for hybrids driven mostly at speed. Your trip computer is more convenient and often optimistic by a few percent, so calibrate it against a hand calculation once rather than trusting it outright.
- Fill the tank completely and note the odometer reading.
- Drive normally for at least a few hundred kilometres, ideally mixing conditions.
- Fill completely again and note how much fuel went in and the new odometer reading.
- Divide the distance by the fuel added for km per litre or miles per gallon, or divide fuel by distance and multiply by 100 for litres per 100km.
- Repeat across two or three tanks and average, since a single measurement is sensitive to how the pump cut off.
What actually changes consumption
The first two are the ones worth acting on for a long trip. Dropping motorway speed slightly and removing an unused roof box together can change a long journey's fuel bill by a noticeable margin, and both are free.
| Factor | Typical effect | Notes |
|---|---|---|
| Motorway at 120 vs 100 km/h | 10-20% worse | Air resistance rises with the square of speed |
| Roof box or bars fitted | 10-25% worse | Worst at higher speeds; remove when unused |
| Tyres 20% under-inflated | 2-4% worse | Also increases wear and affects handling |
| Air conditioning in city traffic | 5-10% worse | Minimal effect at motorway speed |
| Extra 100kg of load | 1-2% worse | More noticeable in stop-start driving |
| Cold engine, short trips | 20%+ worse | Engine spends the journey below efficient temperature |
Comparing driving with the alternatives
Fuel is only part of the cost of driving, which makes a fuel-only comparison against a train ticket misleading.
For a fair comparison, add tolls, parking at the destination, and a per-kilometre allowance for wear — tyres, servicing, depreciation. That allowance varies widely by vehicle, but adding something is more accurate than adding nothing.
Then divide by the number of people travelling. Driving alone is frequently more expensive than a train; driving with three passengers usually is not. This is where the comparison usually turns, and it is why a single answer to 'is driving cheaper' does not exist.
Electric vehicles use the same structure
A car using 17 kWh per 100km over 640km needs 108.8 kWh. At a home rate of 0.24 per kWh, that is 26.11. At a public rapid charger at 0.62 per kWh, the same journey costs 67.46.
The spread between home and public charging is far wider than the spread between fuel stations, so where you charge affects the answer more than which vehicle you drive. Charging losses of roughly 10% also mean you pay for more kWh than reach the battery, which a calculation based on consumption figures alone will miss.
Cost = (Distance in km / 100) x (kWh per 100 km) x Price per kWh
Why trip fuel estimates come out too low
- Using the manufacturer's official consumption figure, which is measured under standardised conditions that do not match ordinary driving.
- Mixing units — US gallons and imperial gallons differ by about 20%, which is a large error on a long trip.
- Forgetting the return journey when planning a budget.
- Assuming fuel prices are constant along the route. Motorway service stations are routinely more expensive than towns a short detour away.
- Comparing fuel cost alone against a full ticket price for another mode of transport.
- Ignoring that consumption is worse on the outbound leg of a loaded holiday trip than on the empty return.
Assumptions and limitations
The formula assumes a single average consumption figure across the whole journey. A trip that is half city driving and half motorway has two quite different rates, and if the split is substantial it is more accurate to calculate each segment separately and add them.
It assumes a fixed fuel price. On a long trip you will likely refuel more than once at different prices, so the result is an estimate built on the price you happened to use.
And it excludes everything other than fuel or electricity. For deciding whether a specific trip is affordable, fuel alone is usually the relevant number. For deciding whether to own a vehicle at all, it is a small part of a much larger calculation involving insurance, servicing, depreciation and financing.
Use your vehicle's real consumption when possible
A trip fuel estimate is more useful when fuel economy comes from your own driving history rather than the manufacturer's headline figure. Record fuel added and distance travelled across several trips, then use the resulting average. For planning, it is also sensible to test a higher-consumption scenario for heavy traffic, hills, weather, cargo or aggressive driving.
Frequently asked questions
How do I convert between mpg and litres per 100km?
Divide 235.21 by the US mpg figure to get litres per 100km, or use 282.48 for imperial gallons. The relationship is inverse, so the conversion works in both directions with the same constant.
Is my car's trip computer accurate?
Usually close but often optimistic by a few percent. Check it once against a full-tank hand calculation and note the offset, then keep using the computer with that correction.
Does driving slower always save fuel?
Only up to a point. Most vehicles are most efficient somewhere around 70-90 km/h; below that, efficiency falls again. The clear saving is in reducing high motorway speeds, where air resistance dominates.
How should I budget fuel for an unfamiliar route?
Add 10-15% to the calculated figure. Detours, traffic, terrain and higher prices along the way all push the real cost above a clean point-to-point estimate.
Should I use the advertised fuel economy?
It can be a starting point, but your actual consumption may differ because of speed, traffic, temperature, load, tyres and driving style.
Do electric vehicles use the same cost structure?
The units differ, but the structure is similar: energy used per distance multiplied by the energy price, plus any applicable charging costs.
Conclusion
Fuel cost is distance divided by efficiency times price, and the only input worth spending time on is the efficiency figure. Measure it yourself across two or three tanks rather than using the official number, calculate each leg separately if driving conditions differ substantially, and add a margin for an unfamiliar route. When comparing driving against another option, include tolls and parking and divide by the number of travelling, because that is usually what decides the answer.
Figures in this article are illustrative. Results depend on your own rates, fees, taxes and circumstances, and are for educational and planning purposes rather than financial advice.