The first thing to examine would be your driving style and circumstances. Cars use the most fuel during their initial acceleration, so if your driving environment and circumstances involve a lot of stops it is only natural for any car to get poor fuel economy. In addition, if you choose to accelerate hard you will also hurt your fuel economy.
In the past a tune up was often performed to try to restore fuel economy. This was a maintenance procedure and not a repair. Cars had many mechanical systems that would wear and move from their ideal factory settings. Restoring those factory settings would restore performance and fuel economy. Common procedures included ignition points, timing, carburetor adjustments, and filter replacements. Some of these procedures were required as often as every 10,000 miles.
Electronic ignition did away with points, and also reduced the need for timing adjustments. As points wore down timing would change. In addition the ignition coil would not develop adequate voltage for proper performance. This loss in performance would also cause poor fuel economy.
Carburetors sometimes required adjustments and if this was not done correctly it could also result in poor fuel economy. Cars run better if the mixture is a little rich. This means a little more fuel in relation to the amoiunt of air being consumed by the engine. Its much like the difference between apple pie and pie ala mode. You may not need the extra richness of the ice cream, but you usually like it better. The same holds true for an engine. The rich mixture makes the engine run smooother, but also hurts fuel economy and emissions.
Modern cars do not go out of adjustment like cars once did, so a "tune up" is not a true option for improving fuel economy. While electronic ignition was good, distributorless ignition is even better. Timing is carefully monitored electronically and rarely moves from the factory adjustment. Fuel injection is also used on just about every car made today. Fuel injection requires no periodic adjustments. Fuel injection also meters fuel more precisely and evenly than a carburetor, and atomizes the fuel completly to ensure the most efficient burn.
One myth that must be dispelled is that catalytic converters hurt fuel economy. A properly operating catalytic converter will not hurt fuel economy and has no negative affect on performance. Of course, a clogged converter will hurt performance, but that once again is a defect that may have been caused by other engine defects.
Any defect that would hurt gas mileage will first show up as a performance problem and you would probably already have brought the car to your mechanic.
If your car is experiencing a performance problem, this could be the cause of the fuel economy issue. A good mechanic will look beyond your engine. Brakes, suspension, and transmission problems may also affect your fuel economy.
There is one final point to be discussed on this issue. To reduce emissions gasoline is formulated differently in winter months. This difference in fuel formula will usually result in a 15% reduction in fuel economy.