If your looking at increasing your mpg, a K&N drop in filter will increase a wee tad bit more, maybe 1-2, depending on your driving habits. Far as a tune is concerned, your throwing money away to something that wont benefit mpg. A tune will increase more pulse width injection on your fuel injectors, so meaning, they stay on more milliseconds than stock with the tune. Your factory PCM has 15 driving habits it learns located in the fuel trim (short term and long term). It relies on 1 of 15 during your drive and your initial 5 minutes of driving averagely, your car is trying to achieve closed loop strategy. Meaning...your cat converter needs to warm up to 600 degrees in order for the 02 sensors to work properly. During the initial 5 minutes, their ignored and your car produces a "rich" condition, shooting more fuel through your engine til your engine temp as well hits normal operational temperature.
So in English (haha), a tune without dyno testing and adjusting your fuel maps (fuel injectors) to run a bit leaner, will cause more fuel efficiency. Hope this helps you. A chip or street bought tune has no true reading of your drive to and back, driving habits, or your sea level/barometric pressure.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille