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Typically speaking that is the amount of MPG that you will see with that car , mileage and age. You can do a good tune up (oil filter, fuel cleaner , fuel filter, plugs, wires, distributor cap, rotor, and so on.) Changing Oxygen sensor may help, however it really depends on how badly they are worn. If the oxygen sensors are worn badly then it will improve the MPG.
Also consider getting a K&N air filter which will also help.