I'm going to throw some old Hot Rodding thoughts out here. First off, if you're currently running in the optimum RPM range for your engine, then it's probably doing as well as you'll get.
Every pound of dead weight you're carrying around is hurting your mpg, take a look at anything you're carrying that isn't needed.
The wider your tires are, the more friction and drag you're creating. If you've changed out your tires for looks, that also can affect mpg.
Keep your tire pressure on the high side of what's listed for them. It also can affect mpg.
Finally another way to alter gear ratio is the overall diameter of your tires. If you go with a taller, narrower tirer it's the same as altering your gear ratio. Problem though could be if your engine has to work harder to get to speed, you negate the added benefit of the taller tire.
The less stop-and-go, the better your mileage (less gas used to get you back in motion).
Drive like you have an egg between your foot and the footfeed.