Km per litre:
measure real mileage and turn it into cost per km

The tank-to-tank method, step by step

Mileage in India is quoted in kilometres per litre, and the only measurement that does not depend on the car's computer is what you pour in at the pump. The tank-to-tank method uses that. It takes two fill-ups and a trip meter, and it works for any petrol, diesel or CNG vehicle if you use the right unit (kg for CNG).

1. 1Fill the tank at the pump until the nozzle clicks off for the first time, and do not top up.
2. 2Reset trip meter A to zero, or write down the odometer reading.
3. 3Drive as you normally would until the tank is at least half empty, preferably close to a quarter.
4. 4Return to the same pump if you can, fill until the first click-off again, and note the litres on the receipt.
5. 5Divide the distance on the trip meter by the litres you just added.

The litres added at the second fill-up are the fuel used during that distance, because the tank started full and ended full. Whatever the fuel gauge claimed in between does not matter.

The formula and the unit family

Europe and several other markets express the same thing the other way, as litres per 100 km. To convert, divide 100 by the km/L figure. A car doing 14.5 km/L uses 100 ÷ 14.5 = 6.9 litres per 100 km.

Be careful when comparing: a higher km/L is better, while a lower L/100 km is better. A calculator that shows both is useful when you read a foreign review of a car you plan to buy.

Worked example: a 412 km stretch

A hatchback is filled to the first click-off with the trip meter at zero. A week later the trip meter reads 412.0 km, and the refill takes 28.4 litres.

Now turn it into money. Assume a pump price of ₹100 per litre (use the current price in your city). The cost of each kilometre is 100 ÷ 14.5 = ₹6.89, so a 12,000 km year costs about ₹82,800 in fuel at that price.

Brochure mileage and road mileage

The claimed figure on a brochure is measured on a standard test cycle with a specified speed profile and no air-conditioning load, so it works as a comparison between cars, not as a prediction for your commute. Real-world numbers usually sit 15 to 25 percent below it.

City traffic, short trips and idling at signals pull the average down. Steady highway driving at moderate speed usually returns the best figure. If the pattern of your driving is mostly short hops, expect the city figure rather than the highway number.

What a small mileage gain is worth

Mileage improvements look small in km/L but are easier to see in litres saved. Consider a driver who covers 12,000 km a year. At 15 km/L the car burns 800 litres. At 18 km/L it burns 667 litres, a saving of 133 litres. At ₹100 per litre that is about ₹13,300 a year.

The returns diminish as the figure rises. Going from 12 to 15 km/L saves 200 litres over the same distance, but going from 18 to 21 km/L saves only 95 litres. Fixing a poor result is worth more than polishing a good one.

• Check tyre pressure monthly, because under-inflated tyres raise rolling resistance.
• Remove roof racks and unneeded load.
• Replace a clogged air filter and keep to the service schedule.
• Avoid long idling; switching off at a long signal pays back quickly.

Comparing fuels by cost per kilometre

Mileage figures only compare like with like, and the unit changes with the fuel: km/L for petrol and diesel, km/kg for CNG, km/kWh for an electric car. The way to put them on one footing is cost per kilometre, which is the price of one unit divided by the kilometres you get from it.

As an illustration with assumed prices that you should replace with local ones: petrol at ₹100 per litre and 14.5 km/L costs ₹6.9 per km; diesel at ₹90 per litre and 20 km/L costs ₹4.5 per km; CNG at ₹75 per kg and 22 km/kg costs about ₹3.4 per km. The lower running cost has to be weighed against purchase price, servicing and the availability of fuel on your routes.

When your figure looks wrong

If a tank shows a result that is far from the others, check the process before the car. Filling at a different pump, a sloping forecourt that lets the nozzle click off early, or topping up after the first click all change the litres by a few percent. A missed fill-up also doubles the apparent distance.

A sudden fall of more than 10 percent across several tanks, without a change in how you drive, is worth a check. Typical suspects are tyre pressure, dragging brakes, a dirty air filter, or a sensor fault. A workshop can scan the car and confirm.