How to calculate your car's fuel economy (km/L, mpg and L/100km)

Fuel economy is the distance a vehicle covers on a unit of fuel. To calculate it, fill the tank, note the odometer, drive normally, fill the tank again, and divide the distance travelled by the fuel it took to refill. That is the tankful method, and it is the only way to get a figure you can trust: the trip computer on the dashboard estimates, the brochure figure was measured in a lab, and your memory rounds up.

The tankful method, step by step

1. Fill the tank completely and write down the odometer reading. Call it reading A. Stop at the first automatic click of the pump nozzle every time, so "full" means the same thing on both fills.
2. Drive as you normally would until the tank is somewhere between a quarter and half full. The longer the stretch, the more the result smooths out traffic and short trips.
3. Fill the tank completely again and write down the odometer reading (B) and the amount of fuel the pump put in (F).
4. Divide the distance by the fuel: (B − A) ÷ F.

Everything else is units.

Worked examples

Kilometres per litre (India, most of Asia, Latin America). Reading A is 42,180 km. Reading B is 42,600 km. The second fill took 28 litres. Distance is 420 km. 420 ÷ 28 = 15 km/L. In India this is simply called the car's average: "the Swift gives 15 in the city."

Miles per gallon (United States). 310 miles on 11.2 US gallons. 310 ÷ 11.2 = 27.7 mpg.

Miles per gallon (United Kingdom). The UK gallon is larger (4.546 litres against 3.785). 310 miles on 9.3 UK gallons is 33.3 mpg, which is why the same car scores higher mpg in UK tests than in US ones.

Litres per 100 km (Europe, Australia). Here lower is better. Take the litres, divide by the distance, multiply by 100. 28 ÷ 420 × 100 = 6.7 L/100km.

Converting between units

From	To	Do this
km/L	L/100km	100 ÷ km/L
L/100km	km/L	100 ÷ L/100km
km/L	mpg (US)	× 2.352
km/L	mpg (UK)	× 2.825
mpg (US)	km/L	÷ 2.352
mpg (US)	mpg (UK)	× 1.201

Quick sanity check: 15 km/L ≈ 35 mpg (US) ≈ 42 mpg (UK) ≈ 6.7 L/100km.

Why your dashboard figure is wrong

The trip computer does not measure fuel. It estimates it from how long the injectors are open and multiplies by an assumed flow rate. Across a tank the error is typically 5 to 10 percent, almost always in the car's favour. Dealers and manufacturers know this, which is why "real-world" tests use the tankful method and a measured pump.

The brochure or ARAI figure is a different problem: it is measured on a dynamometer under a fixed drive cycle, with no air-conditioning, no passengers and no traffic. Treat it as the best case, not the expectation.

From fuel economy to cost per kilometre

Fuel economy tells you how efficient the car is. Cost per kilometre tells you what it does to your wallet, and that is the number that answers "should I take the car or the scooter?" and "is the CNG kit paying for itself?"

Cost per km = price per litre ÷ km/L.

At ₹105 a litre and 15 km/L, a kilometre costs ₹7.00. The same car on CNG at ₹76 a kilogram and 20 km/kg costs ₹3.80 per kilometre. In US terms, gasoline at $3.60 a gallon and 27.7 mpg is 13 cents a mile.

Once you have the cost per km, multiply by your monthly distance and you have the fuel line of your running cost. Add service and you have what the car actually costs to drive, which is where most people get a surprise.

Getting a figure you can trust

• Log every fill. A missed fill merges two stretches and hides what changed between them.
• Fill to the first click, every time. Topping up to the brim on one fill and not the next shifts the figure by a litre or more.
• Use the odometer, not the trip meter, unless you are disciplined about resetting it at every fill. Odometer readings cannot be forgotten.
• Look at the trend, not one tank. Any single tank can be skewed by a highway run or a week of school drop-offs. Three or four fills tell you the truth.
• Keep partial fills in your log for cost tracking, but calculate economy only between full fills.

What a change in the number is telling you

A steady fall in fuel economy with no change in driving usually points at something mechanical or something cheap to fix: tyre pressure down by a few psi, a clogged air filter, old spark plugs, a dragging brake, or a wheel alignment after a pothole. A sudden drop after a service visit is worth a conversation with the garage. A seasonal dip in winter is normal: cold engines, denser air and heaters cost a few percent everywhere.

That is the real value of tracking. The number itself is interesting once. The change in the number is what saves you money.