# CNG vs Petrol in 2026: The Rupees-Per-Kilometre Math Nobody Shows You at the Showroom > CNG looks cheaper at the pump, but the real number is rupees per kilometre, not price per kg. Here's the full math, including mileage drop, conversion cost, and fill-up hassle. Published: 2026-08-05 · Bharometer Blog · https://bharometer.com/blog/cng-vs-petrol-cost-per-km-rupees-math-2026.html CNG costs less per kg than petrol costs per litre. That much is obvious. What the showroom doesn't walk you through is how mileage differences, conversion costs, and the practical limits of CNG networks change the actual rupees-per-kilometre number, sometimes dramatically. Every conversation about CNG vs petrol eventually collapses into two numbers: "CNG is ₹90 a kg" and "petrol is ₹105 a litre." Then someone says "so CNG is clearly cheaper" and the conversation ends. That's the wrong comparison, because you don't pay per litre or per kg, you pay per kilometre driven. The fuel price is only one input. Mileage is the other, and the two fuels don't deliver the same mileage from the same engine. Let's build the comparison properly, using a typical mass-market hatchback as the reference vehicle. The math works the same way for a sedan or an autorickshaw; only the mileage figures change. ## The Basic Formula Cost per kilometre (₹/km) = Fuel price ÷ Mileage in that fuel mode. For petrol: if a hatchback returns 16 km/l in mixed city-highway driving and petrol costs ₹105/l, the cost is ₹105 ÷ 16 = ₹6.56/km. For CNG: the equivalent figure uses km/kg, not km/l. The same hatchback on CNG typically returns 20–24 km/kg in mixed driving (factory CNG kits from April 2025 onward tend toward the higher end; older conversions and original-equipment factory fitments vary). At ₹90/kg and 22 km/kg, the cost is ₹90 ÷ 22 = ₹4.09/km. That's a 38% saving per kilometre at those figures. But those figures aren't guaranteed, and they shift depending on which city you're in, how you drive, and what happened to your engine's petrol mileage after the CNG kit went in. ## Why CNG Mileage Is Harder to Verify Than Petrol Mileage With petrol, a tank-to-tank fill is a clean measurement: litres in, kilometres driven, divide. With CNG, there's a complication. CNG is stored as a compressed gas; the pressure gauge in your cabin is not linear. A gauge reading "half" doesn't mean half the tank's range remains. You'll typically find the range drops faster in the last quarter of the cylinder than the first half suggests. The reliable way to measure CNG mileage is the same tank-to-tank logic, but you need two consecutive full fills at the pump (until the dispenser clicks off), with the odometer reset between them. Only then can you divide kilometres by kilograms and get a real number. Most drivers skip this and rely on the trip computer, which on CNG vehicles is often calibrated for petrol and gives nonsense km/kg readings. For a walkthrough of why the trip computer misleads you, see the breakdown in Why Your Car's Dashboard Average Lies (By 5 to 15 Percent), and the Pump Method That Doesn't. ## The Mileage Drop After Switching Modes Factory-fitted CNG variants are calibrated for CNG from the ECU up. Retrofit CNG kits are a different story: the ECU is still tuned for petrol, and the CNG kit adds its own controller that doesn't always talk perfectly to the existing fuel management system. The result is that many retrofit CNG cars return 10–15% less km/kg than the factory variant of the same car would. Run that through the math. If the factory variant returns 23 km/kg but your retrofit car returns 20 km/kg at the same ₹90/kg, your cost climbs from ₹3.91/km to ₹4.50/km. Still cheaper than petrol at ₹6.56/km, but the gap is smaller than the headline figures suggest. There's a second mileage question: what happened to your petrol mileage after the retrofit? Many drivers report a 5–8% drop in petrol km/l once a CNG kit is installed, because the kit adds weight (the cylinder is 25–40 kg typically) and the ECU recalibration is imperfect. If your petrol mileage falls from 16 to 14.8 km/l, your petrol cost per km rises from ₹6.56 to ₹7.09. That actually makes the CNG savings look larger by comparison, but it also means your fallback mode (petrol, for when CNG pumps are closed or queues are long) costs more than it used to. ## Amortising the Conversion Cost A government-approved CNG retrofit kit, fitted at an authorised workshop, costs roughly ₹18,000–₹22,000 all-in for a typical hatchback. Factory-fitted CNG variants cost ₹80,000–₹1,20,000 more than the equivalent petrol variant (price premium varies significantly by model and trim). For a retrofit: say you spend ₹20,000 and currently pay ₹6.56/km on petrol. On CNG at ₹4.09/km, you save ₹2.47/km. To recover ₹20,000 at ₹2.47 savings per km, you need to drive 8,097 km on CNG before the kit pays for itself. At 1,200 km/month (a reasonable mixed-use city driver), that's about 6.7 months. At 600 km/month (an occasional-use second car), it's over a year. The breakeven is not magic, it's entirely a function of how much you actually drive on CNG. For a factory CNG car, the math is identical in structure but harder: you're comparing two purchase prices and the total cost of ownership over 3–5 years. If you drive 18,000 km/year and 70% of that is on CNG (roughly 12,600 km/year on CNG), the annual saving at ₹2.47/km is about ₹31,122. An ₹80,000 premium pays back in about 2.6 years of sustained CNG use. A ₹1,20,000 premium takes nearly 4 years. These breakevens assume fuel prices stay roughly flat. In practice, CNG prices have moved with natural gas costs, they are not as stable as many buyers assume when they sign the factory CNG order form. ## The Hidden Cost: Time and Network Availability Petrol pumps in India are ubiquitous. CNG stations are concentrated in metros and certain state highway corridors: NCR, Mumbai-Pune, Ahmedabad, Surat, and parts of UP and Haryana have dense networks. In Tier 2 and Tier 3 cities, CNG availability is patchy; in rural areas, it can simply be absent. Peak-hour queues at CNG pumps in Mumbai or Delhi can run 15–25 minutes. If you fill up four times a month and lose 20 minutes each time, that's 80 minutes a month spent waiting. How you value that time is personal, but it belongs in the comparison. A driver filling petrol in 5 minutes versus a CNG driver spending 20 minutes at a pump is making a tradeoff that doesn't show up in the ₹/km calculation. There's also the range consideration. A typical CNG cylinder on a hatchback holds 8–9 kg, which at 22 km/kg gives roughly 175–200 km per fill. The petrol tank on the same car gives 400–450 km. In practice, bi-fuel CNG cars switch to petrol when CNG runs out, so you're never stranded, but if you frequently drive routes with sparse CNG infrastructure, you'll run on petrol more than you planned, and your actual blended ₹/km will be higher than the pure-CNG figure. #### Bharometer Track CNG fills in kg and petrol fills in litres separately, see your real km/kg and km/l, and watch the blended ₹/km across both fuels in one place. [Get the App →](https://bharometer.com) ## Tracking a Bi-Fuel Car the Right Way A CNG car that also runs on petrol creates a tracking problem: your total distance is split between two fuels with different prices and different mileage figures, and both are changing over time. The only way to know your real blended cost per km is to log every fill of both fuels separately and let the math tell you where you actually stand. Bharometer handles CNG and petrol as separate fuel types for the same vehicle, tracking km/kg for CNG fills and km/l for petrol fills. Each fill-up logged gives you the tank-to-tank figure for that mode. Over time, the monthly summary shows you the blended ₹/km, what you're actually paying per kilometre across both fuels combined, weighted by how much of each you used. That number is the one worth watching, not the pump price. The app also covers dual-fuel vehicles (where both fuels run simultaneously, as in some CNG kits), and logs CNG in kg with km/kg mileage automatically. City-level daily fuel prices are included, so the cost calculation uses your city's actual rate rather than a national average. For a broader look at how city-level price variation affects your running costs, the earlier piece on why petrol prices reset every morning and vary by city is worth reading alongside this one. ## A Worked Example: Two Years of Ownership Consider a driver in Pune who drives 1,500 km/month. After fitting a ₹20,000 retrofit CNG kit, 65% of their distance (975 km) runs on CNG and 35% (525 km) runs on petrol. - CNG: 975 km at ₹4.09/km = ₹3,988/month - Petrol: 525 km at ₹7.09/km (post-retrofit mileage drop factored in) = ₹3,722/month - Total: ₹7,710/month on fuel Before the CNG kit, at 16 km/l and ₹105/l, 1,500 km cost ₹9,844/month. The saving is ₹2,134/month. The ₹20,000 kit pays back in 9.4 months. But: in month 3, the driver takes a road trip to Mahabaleshwar, 260 km each way. CNG availability on that route is sparse, so the entire 520 km runs on petrol. That month, the CNG share drops to 40%, petrol share rises to 60%, and the monthly fuel bill rises to ₹8,900, still cheaper than the pre-CNG baseline, but the savings are smaller. Logging every fill tells you this; guessing doesn't. ## The Number to Watch CNG is cheaper per kilometre than petrol for most urban driving patterns in cities with good CNG infrastructure. That is genuinely true at current prices. But "cheaper" ranges from a marginal 15% saving (retrofit car, low CNG usage share, older kit) to a substantial 40%+ saving (factory CNG, dense network, high annual mileage). The showroom will quote you the best case. Your actual number depends on your driving pattern, your city, and what your specific vehicle returns on both fuels. The only way to know which case you're in is to measure it. Log the fills, track both fuels, and read the ₹/km your vehicle actually produces. That number, calculated over 3–4 tank-to-tank cycles, is the one that tells you whether the conversion paid off, and whether it's continuing to pay off as prices change. For a refresher on the underlying measurement method, the guide on tank-to-tank as the only real mileage number covers the mechanics in full. --- Bharometer is a free fuel, mileage and running-cost tracker for India (iPhone). https://bharometer.com