Well, good on getting just over 22 mpg.
Especially considering the speeds you're driving at.

How to up the mpg...

Pump up tires to whatever is the max on sidewall labeling. "Max sidewall" in mpg-fiend lingo.

Reduce speed, as much as you're willing. These cars have a lot of air drag compared to most modern ones (reportedly a c/d of .40 but I have no confirmation on that).

Anyway, the energy needed to overcome air resistance goes up dramatically as you increase speed. All other things being equal, the lowest speed that will still keep you in the highest gear will give you best mpg's.

And of course that assumes you're keeping the car in a good state of tune, good oxy sensor, cat converter not blocked, etc. etc.

For a couple good discussion boards re. fuel economy, see
cleanmpg.com and
gassavers.org
At both of those sites I'm registered as brucepick; you can see Sven's mileage log on cleanmpg.com
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.