Petrol vs Diesel vs CNG: Which Used Car Makes Sense for Kolkata?

The best fuel for a used car is the one that suits how much you drive, where you drive and how long you will keep the car. A fuel that is cheaper per kilometre does not help if the car costs far more to buy or to maintain, or if you drive too little to benefit. This guide compares petrol, diesel and CNG in plain terms, shows a method to test your own case, and points out the questions to ask on each type of used car. Fuel prices change, so none of the figures below are current prices: check today's rates and use the fuel cost calculator with your own numbers.

The short version

These are general patterns. They vary by model, age, condition and how the car was driven.

Purchase cost

Diesel versions of a model usually cost more than the petrol version, new and used. A used diesel therefore has to save you enough in fuel to make up the difference before you sell it. A CNG car costs more or less than the petrol equivalent depending on whether it is a factory fitment or a later conversion. A conversion done properly, with an approved kit and an updated RC, can be fine. A conversion done carelessly can be a problem.

Fuel cost: a method you can use

You need three figures: the car's real-world fuel economy, the fuel price, and how far you drive.

1. Cost per kilometre = fuel price per litre (or kg) ÷ kilometres per litre (or kg).
2. Yearly fuel cost = cost per kilometre × kilometres per year.
3. Break-even distance = extra purchase price of the dearer car ÷ saving per kilometre.

A worked example (illustrative numbers only)

Suppose, for the same model, the diesel car costs ₹60,000 more than the petrol one. Petrol costs ₹9 per kilometre to run and diesel ₹6.50, so you save ₹2.50 per kilometre.

• Break-even distance = 60,000 ÷ 2.50 = 24,000 km.
• If you drive 12,000 km a year, you break even in two years. If you drive 4,000 km a year, it takes six years, which is probably longer than you will keep the car.

Replace these numbers with real prices from PPAC and your own driving figures. The calculation also ignores that diesel cars can cost more to service, so be conservative.

Maintenance and reliability

• Petrol: the simplest to maintain, with parts and mechanics everywhere. Older petrol cars are cheaper to keep running.
• Diesel: strong low-speed pull and good economy, but newer diesel cars have emission-control equipment that can be expensive to fix, and diesels suffer if they are used only for short trips. Ask about the service history, and about injector, turbo and particulate-filter work.
• CNG: the gas system needs its own checks. Cylinders must be tested at the intervals the rules require, and the kit should be approved and recorded on the RC. Ask for the cylinder test certificate and the kit paperwork, and have the system checked for leaks.

Driving pattern

• Short city trips and stop-and-go traffic: petrol is usually the easiest choice. Diesel does not like many short trips, and the saving rarely pays back at low distances.
• Long daily commute or highway driving: diesel can make sense when you drive a lot, if the extra purchase price is covered by fuel savings. CNG can win on cost if you can fill up conveniently.
• Very high daily mileage (taxi-like use): CNG is popular for the running-cost saving, but remember reduced boot space and the need to plan around stations.
• Occasional use: a petrol car with low kilometres is hard to beat on total cost.

Ownership period

The longer you keep a car, the more the running cost matters; the shorter, the more the purchase price and resale value matter. Over a short period, a cheaper petrol car often wins. Over many years and high distances, a more efficient fuel can pay back. Our ownership cost calculator lets you test different periods.

Rules and the environment

Rules about older vehicles and about fuel have been changing in India, and they can differ between cities. Before you buy an older petrol or diesel car, check the current age and emission rules for Kolkata and Howrah with the West Bengal Transport Department or your RTO, and read the PUC certificate. If a car's age is close to a limit, ask what happens when it is reached. Emissions also differ between cars: newer cars meet stricter standards, which is one more reason to ask for the date of first registration and to read the RC.

Questions to ask for each type

If it is a petrol car

• How was it driven? Many short trips can mean extra engine wear.
• Is there a service history? When was the last service?

If it is a diesel car

• When were the injectors, turbo and any emission equipment last serviced or replaced?
• Was it used mainly for long runs or for short trips?

If it is a CNG car

• Is the CNG kit factory-fitted or a conversion? If a conversion, who did it, and is it recorded on the RC?
• When was the cylinder last tested? Can I see the certificate?
• Where is the cylinder, and how much boot space is left?

How to decide

1. Estimate your yearly distance honestly.
2. Check today's fuel prices and the real-world economy of the models you like.
3. Work out the break-even distance for the dearer fuel.
4. Compare it with how long you will keep the car.
5. Choose the fuel that wins on total cost and fits how you drive.

Then browse used cars in Kolkata by fuel type, and follow the steps in our complete buyer's guide.