fuel milage, fuel gage on 1997 Oldsmobile LSS

my wife just recently purchased a 97 olds delta 88 lss, and it seems to be getting low milage for a car w/ just under 110,000 miles.. clean engine, properly maintained.. also, the fuel gauge works when it wants to, not allowing us to get a proper read for comparative

The fuel gauge problem is likely an old sender in the tank getting ready to go. Are you basing the fuel mileage prob off of what the seller said it gets at time of sale? Is the clean engine/properly maintained engine statement based off of what they said they did or backed up with documentation? What do you think it should get and what is it getting?
based on my own experience(ex-trucker, self-taught tinker), i would hazard a GUESS of between 19-21 mpg, as a well maintained(docs to prove) engine/vehicle. as for the in tank sender, how hard would it be for me to replace it?
Would need to drop tank for this repair. That mileage seems a bit high for expectations on this engine. But may be totally doable. If you have no cel then would need to make sure the tune related parts are up to date and see what happens when driven and under what conditions. If is mainly stop and go mileage suffers compared to hiway, as you know.