I have been owner of non efi cultus for almost 3 years and the best mileage I got on highways was 16.5 non AC and around 15.5/16 with AC. In city it dropped to 12-13 partial AC.
One of the reason could be 20W50 oil. If you're using that switch to 10W40 only if your engine is in good health, otherwise it may reduce oil if you switch.
Air pressure, driving habits also account for lower mileage. I always drive calm and try to keep my vehicle below 3K RPM.