Engine performance and gas mileage go hand and hand.  If your vehicle is running inefficiently, you will not experience increased gas mileage and could possibly be doing damage to your engine and components.  We can not stress enough how regular maintenance will prevent unexpected and costly future repairs and roadside breakdowns.  Here are some recommended maintenance items.
1. Regular oil changes.  Pushing dirty oil through your engine and components is harmful to the engine. 
2. Air Filter inspection and change as necessary.
3. Fuel injection system serviced every 15,000 miles to prevent carbon build-up and deposits that restrict fuel flow and degrade engine performance.
4. Transmission fluid and filter replaced every 30,000 miles to ensure proper lubrication, preventing damage and ensuring smooth performance.
5. Timing Belts changed every 60,000 miles or as recommended by the manufacturer to prevent unexpected failure that can leave you stranded and cause severe engine damage.
6. Maintenance tune-up performed as recommended by the manufacturer to ensure longevity, optimal performance of your engine, and improved fuel economy.