Tuning up a car that needs it can boost gas mileage by an average of 4 percent, according to FuelEconomy.gov. Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40 percent.
Other maintenance tips: Use the recommended grade of motor oil (not doing so can cost you 1 or 2 percent), and keep your tires aligned.