Hyundai i20 gives a mileage of upto 18.5kmpl on highways while the city mileage is around 13.3kmpl. The mileage of a car decreases by 2-4 kmpl when you drive with A/C. But you are saying that, your car is giving only 8kmpl. Then there is a problem in your which is dragging extra power from the engine. If your mileage decreases only when you turn on the A/C, then might be the problem is with compressor drive. However, we recommend you to take your car to an authorized Maruti service station and make a proper checkup by an experienced technician.
Initially, Hyundai Elite i20 Magna comes with a petrol and diesel engine but a CNG kit can be fitted with the one which has petrol engine. You will have to sacrifice with power to get more fuel economy and if you convert a petrol car into a CNG car, you may loose the warranty on the car and resale value of CNG cars is lesser than petrol cars. It is always recommended to get the CNG kit fitted from the authorized service center so that you can avail the warranty services as well. Moreover, the performance will depend on how the car is driven and maintained.
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How much fuel goes into a car varies on a lot of factors. Now you haven't mentioned how many km did you drive post the low fuel warning light as that could also determine how much fuel goes into the car. Even though the manual says that the tank capacity is 45 litres, the amount can vary by a small margin, give or take 2 or 3 litres. Also filling fuel in the hot day or in the night also makes a difference considering fuel expands during the hot day and contracts when the temperature is cooler. All said and done, make sure you fuel up within 3 to 4 km after the low fuel indicator light pops up.