If running cost is your main concern, and you are stuck with a petrol car, converting to CNG, rather than LPG, will deliver greater savings.

cng vs lpg comparison of savings

Here is why.

CNG is cheaper

CNG costs Rs. 38.35/kg while LPG goes for Rs. 42.7/ltr. So CNG is cheaper to buy, by about 10 percent.

CNG delivers better mileage

CNG is a slow burning fuel. As a result, it deliver better mileage (1-1.5 km more) than petrol or even LPG. However, it also means you lose on power—by some 15-20 percent. LPG has more calorific value—more energy—than CNG so it has little trouble matching petrol when it come to power delivery. In theory, it should actually deliver better mileage too but several owners have pointed out that, LPG, at best delivers the same mileage as petrol. Often, it delivers 10 percent less. Also read: Different kinds of CNG kits: Advantages and disadvantages

So the combination of lower price and better mileage puts CNG ahead of LPG when it comes to running costs. So how do you decide whether to convert your petrol car to CNG/LPG?

When to convert

Let’s first look at the per kilometre running cost assuming that the donor car (petrol car that will be converted to CNG/LPG) returns fuel efficiency of 15 kmpl:

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CNG is significantly cheaper than petrol and about 20 percent cheaper than LPG. Now, if we were to map these per km costs to usage, then depending on the usage, one would save anywhere between Rs, 1,000 per month to over Rs. 2,000 per month by switching to CNG from petrol. Also read: CNG vs. LPG vs. Petrol vs Diesel: A simple 4 step guide to deciding which fuel type works best for you

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So if running cost is the main concern, then CNG wins hands down. However, if total cost is what you are looking at, then the pictures changes.

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When will you recover the cost of the CNG or LPG kit?

Since the upfront cost of installing a CNG kit is substantially higher, one needs to clock over 23,000 km to recover the investment. So if your average monthly mileage is 750 km, it will take about 2.5 years before you start realizing the benefits. With LPG, it will take about 1.5 years.

In a nutshell, the more you drive every month, the more sense CNG makes.