The cheapest fuel saving mod you can do is buying a lighter right shoe...
Other than driving style, an e-fan and gears would net you some discernible mileage increases, most likely, but there's an investment before actual returns are seen and felt in the wallet. I don't have a SC tuner yet, but from what I've read on here, the "fuel mileage" tune is not really productive.
*EDIT* Also make sure that your tires are properly aired (the smaller and lighter the tire, the better for mileage), your plugs are serviceable, fluids are clean and full synthetic and your sensors and throttle body are clean of gunk and carbon buildup. Just general maintenance can yield the best results.
Last edited by Chet Ubetcha; 04-13-2012 at 01:02 AM.