I have a 93 T auto 4by. Gets 15-17. Don't drive it hard but have lots of hills to climb. Always carrying pounds as well. Was in the 13-14 range until I did a MAF cleanup and brazed the O2 sensor boss on the pipe where it was leaking. Have a exhaust system about to die and am looking into headers to get out of the stupid reverse flow crossover from the passenger side, That should help. Lots of buck though. Sure wish there were more options for the 3.0 exhaust as I believe that is the biggest factor on mileage. My 2by 93 3.0 got 18 - although it was 400 pounds lighter.

What to expect - 180 ci motor with a 4000 pound vehicle....In the good old days 18 was a miracle. (soon to be 61yo). In fact 160 K w/o a motor rebuild every 60 K was a miracle. FYI - GM offered oil filters as an option through 1957!!!