Help! 92 Olds 88 terrible gas mileage

I have a 92 olds 88 with the 3.8l engine and about 70,000 miles. It used to average about 19 mpg and get high 20's on the highway. Now it averages about 11.5. I've changed plugs, wires, O2 sensor, air filter, oil, vehicle speed sensor, fuel pump (shop did that change, line was leaking) and thermostat. None of the changes helped. The car seems to run fine and there are no engine codes stored. I am very meticulous about recording mileage even going to the same pump to refill so it seems something must be wrong that I'm missing. I haven't changed the fuel filter or checked any fuel injectors yet. Any ideas on what could be wrong?

Thanks for any ideas you may have. Gas is now $4.05/gallon around town so this one may be going to the landfill soon if I can't sort this out.

Get a full scan done and look for out of range sensor inputs including fuel trim readings, for starters.