The most economical thing to do is a proper tune up. If you lack the tools and/or skill, check around for someone trustworthy and have a pro do it. It will run it's best, and most economical. Or he may tell you that there are some problems that need repair/replace. You may learn that it performs as well as you want it to. Then you can move on to other modifications, like wheels/tires, upholstery, whatever.