Fuel efficiency comparison:
which car really costs less per kilometre

Compare on the same distance

Mileage figures such as 20 and 15 km/L say which car goes further on a litre. They do not directly show how much money one car saves over the other. To see that, fix a distance, convert each car's mileage into litres, and compare the litres or the rupees.

The distance should be one that means something to you: a monthly commute, a yearly total or a regular trip. The same two cars can look close over 50 km and clearly different over 15,000 km.

Think of it as comparing two meal plans by the weekly grocery bill rather than by calories per rupee. The unit price is informative, but what you pay depends on how much you consume, so the distance you drive has to be part of the comparison.

The comparison method

The formula can be extended to a yearly figure by replacing the distance with your annual kilometres. Many owners underestimate this number; a quick way to find it is to subtract the odometer reading from twelve months ago from today's reading.

Worked example: 20 km/L against 15 km/L

Over a year of 15,000 km, car A uses 750 L and car B 1,000 L. The 250 L gap is 250 × 105 = ₹26,250 in fuel, which is the figure that belongs next to any price difference between the two cars.

Seen as a percentage, car B uses 33% more fuel than car A for the same distance: 8.33 ÷ 25 = 0.333. Going the other way, car A uses 25% less than car B. The two percentages differ because they use different bases, which is a common source of confusion in headlines about fuel savings.

When the two cars use different fuels

Mileage in km/L is not comparable across fuels without including price, because a litre of diesel and a litre of petrol cost different amounts. The cleanest method is cost per kilometre, which is the price per litre divided by the mileage.

The prices above are placeholders for illustration. Substitute the current prices in your city, since fuel prices differ between states and change over time. CNG is sold by the kilogram, so the same method applies with km/kg and price per kg.

Do not forget the way you drive. A car with a low rated efficiency driven gently can beat a more efficient car driven hard, so real-world reviews and your own tank-to-tank figures should override the brochure numbers wherever you have them.

Why km/L misleads at the high end

Fuel used per distance is the inverse of km/L, so equal steps in mileage do not give equal savings. Improving from 15 to 20 km/L over 10,000 km saves 166.7 L. Improving from 30 to 35 km/L, the same 5 km/L step, saves only 47.6 L.

This is why many countries quote consumption in litres per 100 km. Lower is better, and differences translate straight into fuel and money.

The same effect explains why hybrids show such large gains when they replace thirsty cars, while swapping one economical car for another saves little. The biggest rupee savings come from improving the lowest-mileage vehicle in a household, not from polishing the best one.

Converting between units

Converting from km/L to litres per 100 km is 100 ÷ km/L, so 20 km/L is 5 L per 100 km. Going the other way uses the same operation.

Miles per gallon needs care because the US and imperial gallons differ. A US gallon is 3.785 litres and an imperial gallon 4.546 litres, so 20 km/L is about 47 mpg (US) or 56.5 mpg (imperial). Before comparing a car quoted in mpg, find out which gallon was used.

A short conversion list helps when reading foreign reviews: 1 mile is 1.609 km, 1 US gallon is 3.785 L, and 1 imperial gallon is 4.546 L. Convert the whole figure to km/L or L/100 km first, then compare.

Making the comparison fair

• Use the same fuel type or convert both to cost per km, since diesel and petrol are priced differently.
• Use real-world mileage from your own driving where you can, not just brochure figures.
• Compare vehicles driven in the same conditions: highway against city changes both.
• Include the purchase price, insurance and servicing in a longer decision; fuel is only one part.

Test-cycle ratings are most useful for ranking models against each other. The ranking usually holds on the road even when the absolute numbers do not, so a car rated higher on paper tends to be the thriftier one for you as well.

Does the efficient car pay for itself?

A more efficient car often costs more to buy. Divide the extra price by the yearly fuel saving to see the payback time. At the 15,000 km a year and ₹26,250 saving from the earlier example, a car that is ₹1,00,000 dearer takes about 3.8 years to recover the difference in fuel alone.

The result moves with your own driving. At 6,000 km a year the saving is ₹10,500 and the payback stretches past nine years, so for a low-mileage owner the cheaper car can be the better value even with worse efficiency. Resale value, insurance and servicing also differ and belong in a fuller comparison.