CNG vs petrol: the real running cost per kilometre

CNG costs roughly half as much per kilometre as petrol at current Indian prices, and the only honest way to compare the two is cost per kilometre using your own fuel economy figures, because CNG is sold by the kilogram and petrol by the litre. Whether a CNG kit pays off depends on how far you drive each month, how much it costs to fit, and a few costs that do not show up at the pump. Here is the arithmetic and the fine print.

The formula

Cost per km = price per unit ÷ fuel economy.

• Petrol: ₹105 per litre ÷ 15 km/L = ₹7.00 per km
• CNG: ₹76 per kg ÷ 20 km/kg = ₹3.80 per km

Saving: ₹3.20 per km, or about 46 percent. Your figures will differ by city (CNG prices vary more than petrol), by car, and by how the car is driven. Factory-fitted CNG cars usually return 25 to 30 km/kg; aftermarket kits on older cars often less.

Never compare 20 km/kg with 15 km/L and conclude CNG is "better mileage". The units are different fuels, different densities, different prices. Cost per km is the only comparison that means anything.

Payback on the kit

Monthly saving = saving per km × km per month. Payback = kit cost ÷ monthly saving.

Monthly distance	Saving at ₹3.20/km	Payback on a ₹60,000 kit
500 km	₹1,600	38 months
1,000 km	₹3,200	19 months
1,500 km	₹4,800	13 months
2,500 km (cab use)	₹8,000	8 months

The pattern is simple: CNG is for people who drive a lot. Under about 700 km a month the kit takes three years to pay back, by which time you may be selling the car.

The costs that are not at the pump

• Power and pickup. CNG produces 10 to 15 percent less power. Fine in town; noticeable on a loaded highway climb.
• Boot space. The cylinder takes most of a hatchback's boot. For some households this alone decides it.
• Servicing. CNG runs hotter and drier; valve and plug intervals shorten and some garages recommend more frequent oil changes. Budget a few thousand rupees a year extra. The cylinder needs hydro-testing every three years.
• Refuelling time and queues. CNG stations are fewer, queues are longer, and a fill takes longer than petrol. If your nearest station is 8 km off your route, the detour eats into the saving.
• Resale. Factory-fitted CNG holds value well in CNG-heavy cities. Aftermarket kits on older cars can reduce the buyer pool.
• Insurance and registration. Aftermarket kits must be endorsed on the registration certificate and declared to the insurer; the premium rises a little.

Running both tanks honestly

A petrol+CNG car has two fuels with two economy figures and two cost lines. Most fuel apps and spreadsheets treat it as one tank and produce nonsense. The method that works: log each CNG fill in kilograms and each petrol fill in litres, each against the odometer, and calculate economy per fuel between full fills of that fuel. Then compare cost per km side by side. A month in, you will know your real split (most CNG owners run 80 to 90 percent on gas) and your real blended cost.

When CNG pays

• You drive more than about 1,000 km a month and most of it in a city with CNG stations on your routes.
• You can live with the boot and the power loss.
• You will keep the car at least two to three years after fitting.
• You buy a factory-fitted CNG variant where available; the resale and warranty position is far better than an aftermarket kit.

When it does not

• Under 600 to 700 km a month.
• Long highway drives with few CNG stations and a heavy right foot.
• A car you intend to sell within two years.
• A city where the CNG price has crept up to within 40 percent of petrol; check the ratio, not the habit.

Run your own numbers for a month on both fuels before deciding. The pump prices change; your driving pattern is the constant, and it is the thing you can measure.