Maruti Swift and Nissan Micra difference is that micra is 3 cylinder car while swift is 4 cylinder car. Engine Capacity wise both are similar. You can get slightly better average in micra (roughly 1-2 km/litre). Swift is more powerful car as opposed to Micra. Swift being Maruti product will have a much better after sales service, lower cost to maintain.
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/