How to Calculate Your Car’s Fuel Cost per Kilometre
Calculate fuel cost per kilometre using your own fuel receipts, litres purchased and odometer readings, with a worked example.
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Fuel cost per kilometre is a simple way to compare journeys, vehicles and changes in driving habits. It turns a receipt and an odometer reading into a figure you can use in a trip budget.
The key is to measure fuel and distance over the same period; one isolated refill can be misleading if the tank was not filled consistently.
The two numbers worth tracking
Fuel cost per kilometre tells you how much you spent on fuel for each kilometre travelled. Fuel consumption, usually expressed in litres per 100 kilometres (L/100 km), tells you how much fuel the car used over that distance.
They are related, but they are not interchangeable: fuel prices can change while the car’s consumption stays similar.
To calculate cost per kilometre, divide fuel spending by kilometres travelled. To estimate L/100 km, divide litres used by kilometres travelled and multiply by 100.
Keep the units consistent and label the result so you do not confuse dollars per kilometre with litres per 100 km.
A worked example
This example illustrates the calculation only; it is not a benchmark for Australian fuel prices or a claim about typical vehicle efficiency.
375 L/100 km. This is only a reliable estimate if the fuel purchased reasonably corresponds to the fuel used over that distance.
For a practical log, fill the tank consistently, record the odometer and litres, and repeat over several refills.
Use a repeatable method
At each refill, record date, odometer, litres purchased, total cost and whether the tank was filled to the same level. After several refills, add the costs and divide by the kilometres covered during that period.
Avoid comparing a short city run with a long highway trip without noting the difference in conditions.
Your dashboard trip computer can be useful for spotting trends, but it may not match calculations from fuel receipts exactly. Consistent recording is more important than chasing a perfect number from one tank.
A vehicle logbook can make the data more useful by connecting fuel spending to maintenance, trip distance and other ownership costs.
Why the figure changes
Fuel use can rise with stop-start traffic, high speeds, steep terrain, heavy loads, towing, roof racks, tyre pressure and air-conditioning demand.
A change does not automatically mean the vehicle has a fault, but a sudden and persistent increase deserves investigation. Check for obvious causes such as load, pressure and route changes before assuming the engine needs repair.
For fair vehicle comparisons, use similar routes, loads and driving styles. Official consumption figures can help compare models under a standard test, but they do not promise the exact result you will get on your commute.
The Australian Green Vehicle Guide is useful for model-level comparisons; your own records show what happens in your actual use.
Turn the number into a trip budget
15 = A$90. Add a buffer for detours, traffic, terrain and price changes. Include tolls, parking and accommodation separately so fuel does not become a proxy for the whole trip cost.
For a modified 4WD, compare fuel use before and after significant changes such as heavier tyres, roof accessories or additional touring equipment. Record the conditions alongside the numbers.
That makes it easier to understand whether a modification changed consumption or whether the difference came from route, speed or payload.
How to keep a useful fuel log
Record the odometer at each refill and whether you filled the tank to the same point. For a more stable estimate, use the distance between consistent full-tank refills and add the litres purchased during that period.
If you top up irregularly, the fuel bought may not match the fuel consumed over the same kilometres, making the calculation less precise.
Keep notes about towing, roof loads, long idle periods or unusually heavy traffic. A result of 11 L/100 km on a hilly, loaded trip should not be compared without context to 8 L/100 km on a quiet highway.
The note beside the number is what makes the data useful later.
Interpret changes rather than chasing a perfect number
A fuel-cost figure is most useful as a trend. If consumption rises, check whether the route, traffic, load, tyres or towing changed before assuming the car needs a repair.
If the increase persists under similar conditions, ask a mechanic to investigate. Avoid replacing sensors or adding products based only on a high reading.
For a modified vehicle, note when tyres, roof equipment or other accessories change. The record helps separate the cost of the build from the effect of driving conditions.
It also gives you a more defensible estimate for future road trips than a single manufacturer figure.
Frequently asked questions
Should I use one tank to calculate fuel cost?
You can, but several consistent refills usually give a more useful estimate because the amount of fuel remaining in the tank can distort a single period.
Is fuel cost per kilometre the same as L/100 km?
No. Cost per kilometre is money spent per distance; L/100 km measures volume of fuel used per distance.
Why does my result differ from the dashboard?
Trip computers may use different measurement and rounding methods. Compare trends over several refills and use consistent records.
