Sorry to inform you but Nissan Micra doesn't come with automatic transmission variant and company have no plans to launch its automatic variant this year. However, you may go for Maruti A Star or Hyundai i10 which are of the same segment.
Maruti has recently launched the automatic version of A-Star which is powered by the company's well known fuel efficient 1.0L, 998cc, KB Series petrol engine. For detailed information about this automatic variant you may visit the below given link
Maruti A-Star AT VXI
Another option is Hyundai i10, its four variant comes with automatic transmission gearbox. All of these four variants are powered by the 1.2L, 1197cc, Kappa engine. You may visit the below given link for detailed information about these variants.
Hyundai i10 Magna AT
Hyundai i10 Sportz AT
Hyundai i10 Asta AT
Hyundai i10 Asta w/Sun Roof
For comparison between these four variants you may visit the below given compare link.
Compare Hyundai i10 Models
To compare automatic variants of Maruti A Star and Hyundai i10 you may go through above given link.
Compare Maruti A Star and Hyundai i10
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.