I missed the tire size being 265's, I think 225's are stock. BKG is right about the mpg being closer to 15.5-16mpg's.

Did the full tuneup include cleaning the injectors, a new fuel filter and a compression check/valve adjustment?

How old is teh O2 sensor, from what I understand the OBD1 sensors get lazy with age and tend to run rich.

Was the truck getting better mileage before, or is it a new rig for you?

What pressure do you run in the tires?

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed