The fuel gauge is extremely inaccurate and not a good indication of the kind of mileage you will get.
When you buy such an old car you have to catch up to the maintenance schedule which undoubtedly has been neglected. Do a FULL tuneup. Distributor cap, rotor, spark plugs (get NGK or Denso), fuel filter, check base ignition timing, clean out throttle body, IACV, intake manifold, use seafoam in the oil, through a vacuum hose, and in the gas tank, check/replace lazy O2 sensor, check alignment, change oil and filter, transmission oil, change/clean PCV. Check spark plug wires. Replace charcoal canister. Replace in tank fuel strainer. On all my Hondas I like to get the injectors balanced and blueprinted.
You do all this and you should easily achieve 35+ mpg, pick up some pep, and enjoy a reliable, trouble free car for years to come.
Civic VX, D15Z7, 5 Speed LSD, AEM EMS, AEM UEGO, AEM Twin Fire, Distributor-less, Waste Spark