Please dont go confused advises because Hyundai i20 is popular car in India.
The Hyundai i20, 1.4 L, Gamma AT Petrol with DOHC and MPHyundai i20 SpeedometerFI fuel system and In Line type 4 Speed automatic transmission gearbox engine produces a maximum power of 100 PS at 5500 rpm and a maximum torque of 13.9kgm at 4200 rpm. The highly powered engine reaches the breakthrough 100 km in just a span of 15.4 seconds and top speed of 164 kmph.
Hyundai i20, 1.2 L, Kappa PL petrol 5 Speed manual transmission gearbox with In Line type Engine produces a maximum power of 80PS at 5200 rpm and a maximum torque of 11.4kgm at 4000 rpm. This engine crosses the 100 km barrier in just 15.2 seconds with a top speed of 154kmph.
Hyundai i20, 1.4 L CRDI Diesel engine with new 6 Speed manual transmission gearbox performs excellently and produces a maximum power of 90 PS at 4000 rpm and a maximum torque of 22.4kgm at 1750-2750 rpm. This extra high powered engine crosses 100 km in 13.2 seconds and reached top speed of 166kmph.
This is capable to climb on hill roads and for more information about the car please visit the given link as above.
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.