the ecu will not report, 2mpg low. not ever. (LOL) (but if not in closed loop at idle, it will suck fuel at idle. or at cruise, and the scan tool shows you that fact...)
and not putting in 02 sensor at 80-100kmiles, invites this...
why not do the 60k tune up
and the 80k (8v 02s are changed, sooner , a more dirty motor)
buy a scan tool, after the 02 is new, and the mph is low.
see if hit holds closed loop at all times , it must, idle and cruise.
these old year cars dont report a dead heater in the 02, ( it just pops open, not slowly , the heater just burns out, like a flash bulb,, the CELL side, 02 side, does get poisoned in time.)
use an ampclam amp meter see if o2 draws 1amp, key on..
most don't. due to lack of maintenance. (at 80k _miles on 02)
the 02 heater is short lived.... in terms of 200k. on any clock.
that is correct, i to monitor mpg , for only that same reason. something, is up.? (get a scan tool , it will pay for itself, in labor, fuel, and parts thrown wrong)
see first link in my page to old 0bd1
cars here... http://www.fixkick.com/ECU/Authority...OBD2_scan.html
but not in the city, city mpg varies with , well , traffic. (snow, ice,speed, idling times., frustration, hot right foot, etc, YMMV, IIRC. LOL)
you glove box missing the op guide. its free. to all drivers (60,120,180,240.. and so forth)
here is a full book, just for your glove box.. great tips on 4wd too.