Start with a full tune up. No way should a 300zx being getting 15mpg. While doing the tune up also check for vac leaks, corrosion of electrical connections, etc. Chances are if you do the full tune up and inspection of your car you'll not only solve your gas mileage issue but other issues you probably didn't know you had either.
um i believe if the ECT isnt operating properly it thinks the engine is too hot and injects more fuel to cool the operating temperatures more....thus...bad gas..just something else to check besides the list fakina made