Why is CNG mileage measured in km per kg instead of km per litre?

CNG is a compressed gas, so its volume changes with pressure — a litre of CNG is not a fixed amount of fuel. Pumps therefore sell it by mass (kilograms), which is pressure-independent, and the correct mileage unit is kilometres per kilogram (km/kg).

Can I compare a CNG km/kg figure with a petrol km/l figure?

Not directly — they are different units for different fuels. The meaningful comparison is cost per kilometre: divide each fuel's price by the mileage you get on that fuel. For typical Indian prices, CNG's ₹/km works out to roughly half of petrol's, which is the entire economic case for CNG.

How do I track mileage on a bi-fuel (CNG + petrol) car?

Track the two fuels as two separate tanks with separate full-to-full calculations — km/kg for CNG and km/l for petrol. Mixing them in one calculation is meaningless because kilograms and litres can't be added. A per-fuel log also shows each fuel's own cost per km, so you can see your blended running cost.

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CNG mileage explained: km per kg, and how it compares with petrol

Published 9 July 2026 · Bharometer Guides

The short answer

CNG is sold by the kilogram, not the litre, so CNG mileage is km/kg: kilometres driven ÷ kilograms filled, measured full-tank to full-tank exactly like petrol. A typical CNG hatchback delivering 25 km/kg at ₹75/kg costs ₹3 per km — usually around half the per-km cost of the same car on petrol.

Why kilograms and not litres

CNG is a gas compressed to around 200 bar. Its volume depends on pressure and temperature, so "a litre of CNG" isn't a fixed quantity of fuel — but a kilogram is a kilogram at any pressure. Dispensers therefore meter by mass, receipts show kilograms, and the only meaningful mileage unit is kilometres per kilogram. If an app or spreadsheet forces CNG into litres, its numbers are fiction.

Calculating km/kg

The method is identical to the full-tank method for petrol: fill to cutoff, note the odometer, drive, fill to cutoff again, and divide kilometres by kilograms dispensed at the second fill. One CNG-specific caveat: a CNG "full" depends on the station's dispensing pressure — a low-pressure fill holds less than a true full tank. Filling at the same station, or at stations with healthy pressure, keeps readings consistent; treat an obviously short fill as a partial.

Comparing CNG with petrol: use ₹/km

Km/kg and km/l cannot be compared directly — different units, different fuels. Convert both to cost per kilometre:

Petrol: ₹100/l ÷ 15 km/l ≈ ₹6.7/km
CNG: ₹75/kg ÷ 25 km/kg = ₹3.0/km

Same car, same roads — CNG at less than half the running cost. Against that saving sit the cylinder's boot space, slightly reduced power, periodic cylinder testing, and hunting for a working pump on long routes. The ₹/km gap, tracked over months, is what tells you whether the trade-off is paying for itself. Our cost-per-km guide covers the full picture.

Bi-fuel cars: track two tanks, honestly

Most CNG cars in India are bi-fuel — they start on petrol and can switch any time. That means two separate fuel systems with two separate mileages: km/kg for the CNG side, km/l for the petrol side, each with its own full-to-full chain. Adding a petrol fill into a CNG calculation (or vice versa) poisons both numbers, since litres and kilograms can't be summed. A proper log keeps the chains separate and shows each fuel's own ₹/km — plus how much the CNG side is saving you every month.

CNG done right, automatically

Bharometer tracks CNG in kg with true km/kg mileage, supports bi-fuel (CNG + petrol) vehicles as separate tanks, and shows each fuel's cost per km — including how much you're saving versus petrol.