You can start with a super chip which is suppost to increase power and boost fuel economy by up to 15%. A cold air intake will help your motor breath better and increase economy. Also you can get a low restriction exhaust system. All these suggestions will also give you added power and thus your motor will not labor as much to get your car to speed and maintain it's speed. You can also experiment with different speeds, every car has it's "sweet spot" for gas mileage, for some 55mph might be good where as others like 80mph better, it all depends on gearing and HP. Hope this helps.