It totally depends on your requirement that which one you should buy among i10 and i20. If you are looking for an economical car with high fuel efficiency and low maintenance car, than you should go for Hyundai i10, and at the same time if you are looking for a powerful, stylish car which can provide smoother drive, and for which you are willing to spend slightly more, than you can go for Hyundai i20 model.
Both Hyundai i10 and i20 are one of India's best selling premium hatchbacks, with also having a good presence in the foreign markets like Europe and South-East Asia.
For more precise comparison you can click on the link given above- Compare Hyundai i10 with i20.
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.
Click here to find the nearest Hyundai service centre by selecting your city in the drop down list.
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.