How to calculate your car's real fuel cost per km (and why the full-tank method works)

By the TripRank team · 30 October 2026 · 6 min read

Your dashboard's mileage display is an estimate, and the brochure figure is measured on a test rig. The only number you can trust is the one you calculate yourself from fuel you actually bought. It takes two fill-ups and a little arithmetic. This is the full-tank method.

The full-tank method, step by step

1. Fill the tank until the pump stops by itself. Do not top up after the click.
2. Note the odometer reading, for example 24,850 km.
3. Drive as usual. A week or two is plenty.
4. Fill the tank again until the pump stops, and note how many litres went in (say 32.5 L) and what you paid.
5. Note the new odometer reading (say 25,310 km).

Distance driven is 25,310 − 24,850 = 460 km. Mileage is 460 ÷ 32.5 = 14.2 km per litre. If the fuel cost ₹3,250, your cost is ₹3,250 ÷ 460 = ₹7.07 per km.

Why it works, and why partial fills do not

Filling to full both times means the fuel you add at the end is exactly the fuel the car used in between. If you stop short once, that link breaks. A partial fill is still worth logging, though: it adds fuel without closing a stretch, so a good fuel log counts it correctly when you next fill to full.

Common mistakes

• Using a different pump or a different stop point each time. The nozzle click is the best marker.
• Trusting one reading. Average three or four fill-ups, because tyre pressure, air conditioning, load and traffic all move the result.
• Comparing a city fortnight with a highway weekend.
• Forgetting that electric cars use kWh, so the sum becomes kilometres per kWh and cost per km from the tariff.

Keep it in a fuel log

In TripRank, open your car in the garage and tap Fuel log. Enter the odometer, the litres and the price, and tick "Filled the tank" when the pump stopped on its own. The app works out your real mileage, cost per km and how close you get to the ARAI figure. Your trip cost estimates then use your measured mileage instead of a guess. To understand the gap, read why real-world mileage never matches ARAI, and the habits that close it.

Plan trips with the real number

Once you know your cost per km, a road trip budget stops being guesswork. Install TripRank free and start your log with the next fill-up.