http://www.fueleconomy.gov/feg/PowerSearch.do?action=noform&path=1&year1=1989&year2=1989&make=Volvo&model=740&srchtyp=ymm

Our cars are not super efficient by modern standards. Primitive electronics, weight about 3100LB, 3.73 rear gear, old 4 speed automatic transmission. Plus, I read that our fuel tanks are 15.8 gallons, but have never put that much gas in my car even at empty? Anyway, about the most you can get is 24/26 highway in these cars, though I laugh at folks who say they are getting 32MPG :) My gas mileage improved awhile back very noticeably when I replace the exhaust system including catalytic converter. Otherwise, a good tune up, good oil weight, clean throttle body is about all you can do. Oh, 35/36 PSI in tires helps, too.

Sounds kinda low for a full tank. But the most reliable way is fill it up, zero out trip meter. Next fillup, divide miles driven by the gallons it takes to fill up. You didn't say whether manual/auto, turbo/nonturbo? I'm averaging 24mpg manual tranny with turbo. If you have the standard 15gal tank, even at 20mpg, that's close to 300 out of a tank.
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