I'm pretty sure the 88 HF had a 10.6 gallon tank, so it sounds like your gauge is off about 5 gallons. OTOH, 20 miles to the gallon sucks for a Honda... even my RX-7 gets 20MPG, and it needs work. Try a full tune-up, and see what happens from there. If you feel so inspired, check and/or replace the O2 sensor, they have been known to kill milage when they "burn out."