I think that car probably is more in the optimal range of 25 mpg city.
I have a 90 and the original window sticker which says 30 hwy and 24
city, but I don't think I've ever gotten 30 mpg, and I reset the
tripometer at every fill up.
Is you idle above 1000 rpm? If so you could scan the engine for lean
codes, 41, 47, etc. At that high mileage you might have some intake
air leaks, even when the vacuum lines are all good. The intake could
be slightly warped against the head, the throttle body butterfly valve
shaft could be worn, etc. A good smoke test would tell you if
anything, if you can find a place that does that. Haven't done one
myself but I would like to on my car.
Someone correct me if I am wrong, but I think any unmetered are will
signal a lean condition to the computer and it will try to compensate
by shooting more fuel into the engine, resulting in poor fuel economy.