Are you running stock size tires because bigger tires change your speedo and odometer readings and more aggressive tires can eat gas. Properly inflate your tires, stay close to speed limit, tune up the engine if it's been some amount of time since the last one. As far as boltons you can add a cat back and header for exhaust, you can add a cold air intake, you can tune it or add a chip but that's the most expensive option. If you go into the exhaust check out the shape of the cat and the o2 sensors. Also, if you can fill up at big name gas stations they get better quality gas and try to ditch as much weight as possible. But if you are overall worried about gas trade it off for an econobox.