Hi Mr. Sheeba Krishnankutty,
This is in regards to your query posted on the website CarDekho.com, we would like to inform you that Nissan Micra does not comes with CNG kit. CNG kit can be fitted in its petrol variants but it is advisable to get it fit from authorized dealers or Govt. authorized centres. The performance of CNG is quite low.
We encounter similar question many times, the easiest way to increase the mileage is to keep your car healthy. Keep the tyre pressure proper, air filter clean, always follow speed limits and turn off your engine on traffic lights. Find more tricks here- Please find the details Chevrolet service centers here- http://forum.cardekho.com/topic/356-how-to-increase-mileage-of-your-car-tips/
There is not any special petrol recommended for NIssan Micra. You can use simple unleaded petrol in it. However, you can use Speed, Power or Extra premium petrol for better performance of the vehicle.