Changing your driving habits will have a bigger effect than almost anything else you can do, and the best bit is it doesn't cost you a thing.
15, ouch. But I assure you, you can do better. The first thing you have to do is to stop driving like everyone else around you.
All those people are driving with one goal - their intent is to have arrived. In order to improve your vehicle's fuel economy, you must drive it with an eye not toward having finished driving, but how you're going to be driving it in the next ten seconds, and then ten seconds after that, etc. Like most people you probably follow a few specific routes to your usual destinations: work, the store, Uncle Bob's for Sunday dinner, etc. Well, you know them so well, you can avail yourself of advantages. Steep uphill approaching Uncle Bob's? Don't just drive right up it: at some point you can let off the gas and allow yourself to coast to a near-perfect stop, rather than rely on the brakes. You both save a little fuel and a little brake wear that way. It's a smoother ride, too.
See a light in the distance that's green? How far off? If it's far enough, you might be pretty sure it's going to be red by the time you get there. Oh, well, no point in hurrying toward it then, is there? Back off the gas and coast. If you want to hold a little more speed on your coasting, shift into Neutral. If it goes to red before you arrive, you might just be able to coast long enough that it has changed back to green before you get there! Woohoo! I love when that happens.
DON'T turn your engine off for coasting. That's for manuals and only a few automatics. Research carefully before trying it with your machine.
There's a sticky called
100+ hypermiling/ecodriving tips and it's full of good advice. I look forward to seeing how well you do.
Start a garage page and fuel log for your ride. You can't manage what you don't measure, and once you know how well you've done, you can start to maximize what works best.