You may also have a leak in the fuel pressure regulator diaphram. Carefully !!!disconnect the vacuuum line from the regulator while the engine is running. Have some rags handy. There should be no gas coming out of the regulator where the vacuum hose connects. Maybe try running and few tankfulls using some fuel injector cleaner.

The first thing that I would look at is the vacuum lines on the car, if they are cracked and leaking they raise havoc with fuel milage.

How is the perfomance, if it's rather sluggish esp up hills, you may have a knock sensor going bad.

How are the spark plugs, wires, bosh distributor cap and rotor button, air filter and a clean flame trap(if it's not a turbo)and the previously mentioned should bring the car into spec, and give better mileage.

Is the car using any oil, might want to go with a Auto r-x treatment, followed up with a motor vac.

If the car needs all that I've mentioned it's going to cost about 300 bucks and thats with you doing some of the work installing new parts, including having a indy shop doing the motor vac.

First thing though would be the vacuum lines. Hook up a vacuum gauge and get a reading, if under around 17lbs, you have either a vacuum leak or something is retarding your timing, in the later case suspect a knock sensor.
Good Luck
Poolman