Quick answer
CNG fuel cost per kilometre equals CNG price per kilogram divided by mileage in km/kg. At an assumed ₹90/kg and 25 km/kg, it is ₹3.60/km. Include petrol used alongside CNG before estimating actual savings. Recovering a higher purchase price also depends on annual distance and other ownership costs.
For a useful buying decision, compare equivalent petrol and CNG cars using local fuel prices and realistic efficiency. Then divide the extra purchase amount by the net saving per kilometre. If the saving is zero or negative, this calculation does not produce a fuel-cost payback.
Keep kilograms and litres separate
CNG is priced by kilogram in a normal vehicle-fuel calculation, while petrol is priced by litre. A CNG mileage figure uses km/kg. Petrol mileage uses km/l. Do not compare the two fuel prices alone or treat a kilogram as a litre.
At the assumed ₹90/kg and 25 km/kg, ₹90 divided by 25 gives ₹3.60/km. That calculation covers only the CNG used. It excludes any petrol consumption, maintenance, insurance and the cost of buying the car.
Record the unit with every input. A worksheet showing only 90 and 25 can become confusing when reused for another fuel. Clear labels prevent a mathematically correct formula from using the wrong type of price or efficiency.
Use the price for your actual filling location
CNG prices differ between distribution areas and can change. PNGRB publishes a price table identifying the supplier, area, district, revision date and retail price. A dated entry should not be treated as a guaranteed current price at every station.
Confirm the rate at the outlet you expect to use, or through the relevant supplier's current information. PNGRB also provides links to city-gas entities. Keep the area and check date with your calculation so the estimate can be updated.
All numerical fuel prices in the examples below are assumptions. They are not a live rate announcement for Delhi or any other city. Replace them with your verified local values before comparing cars or setting a monthly budget.
Choose a realistic CNG mileage input
Use the efficiency for the exact car and transmission, and distinguish certified figures from your own expected use. Traffic, route, load and operating conditions affect results. An advertised test figure does not guarantee the same efficiency on every journey.
If you already own the car, collect several refuelling records. CNG fills can vary with conditions, so one fill-to-fill figure can be noisy. Look at a longer set of records rather than treating the best single trip as the annual expectation.
Follow the vehicle's approved refuelling process. Do not attempt to force a larger fill or alter equipment to make a mileage test look more consistent. Budget accuracy is never a reason to bypass instructions for a pressurised fuel system.
A fuel only comparison using assumed inputs
This example compares an assumed petrol efficiency of 15 km/l at ₹100/litre with an assumed CNG efficiency of 25 km/kg at ₹90/kg. It is not a specification comparison for named models.
| Calculation | Petrol example | CNG example |
|---|---|---|
| Assumed fuel price | ₹100 per litre | ₹90 per kg |
| Assumed efficiency | 15 km/l | 25 km/kg |
| Fuel cost per km | About ₹6.67 | ₹3.60 |
| Fuel quantity for 1,000 km | About 66.67 litres | 40 kg |
| Fuel cost for 1,000 km | About ₹6,667 | ₹3,600 |
On these assumptions, the fuel-only saving is about ₹3.07/km before including petrol used by the CNG car. Calculations use unrounded values, so multiplying the displayed rounded cost can produce a small difference.
Do not interpret the saving as a universal CNG advantage of the same amount. A different local price, efficiency or vehicle changes it. The purchase premium and non-fuel costs still need their own calculation.
Include petrol used by the CNG car
Some vehicles use petrol during normal starting or changeover, and a driver may also travel in petrol mode. The actual behaviour depends on the vehicle. Maruti Suzuki supplies CNG supplementary manuals alongside its model instructions, so use the relevant one.
Do not assume that every CNG car starts in petrol or that all cars use the same changeover method. Some systems operate differently. The budget should reflect the actual vehicle and your likely use, not a generic rule copied from another model.
For a simple illustrative month, assume 900 km is covered on CNG and 100 km on petrol. At the earlier efficiency and price assumptions, CNG uses 36 kg costing ₹3,240. Petrol uses about 6.67 litres costing about ₹667.
The combined fuel bill is therefore about ₹3,907 for 1,000 km, or ₹3.91/km. The equivalent assumed petrol-only bill is about ₹6,667, so saving becomes ₹2,760 for the month, or ₹2.76/km. This is a mode-distance example, not a measurement of starting fuel use.
Calculate the extra amount paid for CNG
Compare matching equipment and registration assumptions. A higher CNG trim should not be compared with a lower petrol trim without identifying the equipment difference. Otherwise, the entire premium may wrongly be attributed to the fuel system.
Use the final on-road figures after confirmed discounts, matching insurance scope and necessary differences. Keep optional accessories separate. If you compare only ex-showroom prices, label that narrower basis and recognise that it may not equal the extra cash paid.
For the next example, assume an ₹80,000 extra purchase amount. This is not a current price premium for any model. Use your actual pair of written quotes to decide what belongs in the payback calculation.
Work out simple break even distance
Simple fuel break-even distance equals the extra purchase amount divided by saving per kilometre. With the assumed ₹80,000 premium and the blended saving of ₹2.76/km, the result is about 28,986 km.
At 12,000 km annually with the same fuel mix and savings, that is about 2.42 years. At 6,000 km annually, it is about 4.83 years. The car does not become free at that point; the model only shows recovery of the specified initial premium through the assumed fuel saving.
If you plan to sell before reaching the distance, fuel savings alone may not recover the premium. Resale could change the result, but should not be assumed to repay the balance without evidence. Treat it as a separate uncertain estimate.
Allow for differences beyond fuel
Add verified differences in scheduled maintenance, insurance and financing where relevant. CNG equipment can have its own inspection and maintenance requirements. Follow the actual manual and applicable testing instructions rather than removing those costs to improve the payback.
For a yearly comparison, calculate annual fuel saving and subtract the additional recurring costs. If the blended example saves ₹33,120 in fuel at 12,000 km, an assumed extra ₹3,000 annual cost leaves ₹30,120. The ₹80,000 premium then takes about 2.66 years to recover on that simplified basis.
The ₹3,000 is an example input, not a service-cost claim. Financing, changing prices and resale are still outside that simple result. A fuller ownership calculation can include them, but should show its assumptions rather than hiding them behind one payback number.
Station access changes practical value
Check stations near home, work and regular routes. Count the detour distance as part of the travel required to use the fuel. A low pump price can be less attractive if each refill adds a substantial journey.
Consider waiting time too. It may not appear as a direct cash expense, but it affects whether you will actually keep using CNG. If inconvenience leads you to use more petrol, revise the fuel mix instead of keeping the original optimistic saving.
For longer trips, check fuel availability before departure and retain a sensible plan for the vehicle's permitted alternative mode. Do not run a system outside its instructions in an effort to preserve a low cost-per-kilometre target.
Test less favourable assumptions
Calculate what happens if CNG price rises or expected efficiency is lower. Holding all else unchanged, ₹100/kg at 25 km/kg gives ₹4/km, compared with ₹3.60/km at ₹90/kg. The petrol comparison can also move.
Do not assume both prices will change in the same proportion. Use a few clearly labelled scenarios rather than forecasting a guaranteed spread for several years. Lower annual distance can also extend payback even if the per-kilometre saving stays unchanged.
If a scenario produces no positive net saving, do not divide the premium by a negative value and call it a payback period. Explain that the model does not recover the extra amount through those assumed savings.
Check the car as well as the calculator
Try the boot, seating and performance in the actual version. Factory CNG packaging differs between models, so avoid assuming every car loses the same luggage space. A cheaper operating result is less useful if the vehicle cannot carry what your family needs.
Check warranty and servicing support for the system. For a used or converted car, verify documentation, approval and condition through suitable professionals. Do not treat an undocumented conversion as equivalent to a factory version merely because both use CNG.
Safety-related inspection and cylinder requirements should be confirmed from the vehicle documentation and competent providers. This cost guide is not a refuelling, repair or cylinder-testing manual. Never delay required work to make a spreadsheet result look better.
Questions about CNG costs
Is CNG always half the cost of petrol
No. Compare price divided by realistic efficiency for each fuel and include petrol used by the CNG vehicle. The ratio changes with location, car and operating conditions. Avoid a fixed nationwide percentage saving.
How do I calculate monthly CNG spending
Divide the distance covered on CNG by km/kg, then multiply by rupees per kilogram. Add petrol spending separately. If you only know total distance, make a clearly labelled fuel-mix assumption and update it using records later.
Does break even mean all ownership costs are recovered
No. A simple result usually covers only the stated purchase premium and specified savings. It does not repay the entire car price or include every maintenance, finance and resale effect unless those inputs are explicitly included.
Is CNG suitable for low annual running
It can be usable, but the premium may take longer to recover. Check your expected distance and ownership period. Fuel access, luggage needs and the actual quoted premium also matter.
Can I ignore petrol because I mostly use CNG
No. Include any petrol your vehicle and driving pattern require. Use actual instructions and records rather than assuming all CNG systems work the same way. Even a small recurring petrol spend changes the saving.
Compare with your own routes and quotes
Get matching petrol and CNG on-road quotes, verify local fuel prices and estimate a realistic fuel mix. Calculate a normal and a less favourable payback case. Choose the CNG version only when its practical fit and expected savings suit how long and how far you plan to drive.