If you're getting 12-16 MPG then you're very unlikely to get better than that. You could change the gear ratio to a 3.23:1 maybe if you do a lot of highway driving but I doubt that would be worth the cost. The best thing you could do to increase fuel economy would be to learn to drive more efficiently. I can't really tell you exactly how to do that either because it changes depending on where you are (weather, terrain, etc.). You'll just have to try different things and see what works. I can say most people do not drive very efficiently, they drive so slow the engine is always accelerating or they WOT from every red light and race up to the traffic just to slam on the brakes. Smoothness is key.