The 3L v6's weren't noted for power or economy. 10-13 PMG with M/S tires and AT is unfortunately normal. Wasn't EPA estimate like 17 MPG new. I had read that poor mileage was attributed to lockup design (or lack of) in the AT. It wasn't changed until '96 - and then better mileage resulted. Are your tires over sized - 225/75x15 or 31x15 (optional and has different gearing - should have sticker on door) were stock . Oversize tires and knobby tread tires get worse mileage plus throw off speedo/ODO. Cheap attempts to fix are - ensure you have new thermostat, OEM plugs and wires, new air filter, clean injectors with Chevron Techron, over inflate tires by 4# (over specified), avoid fast starts, run synthetic fluids. If you have ADD hubs in front swap them out for manual AISIN or Warn - fewer moving parts. Headers, larger exhaust and free flow CAT would be more costly investments to slightly improve performance/MPG. If ya have 200k miles on it and really like the vehicle (and no rust in rear wheel wheels and tail gate) - a common transplant that costs same as rebuilding current engine is the GM 3.8-4.3L V6.
I had a '90 with the 22RE 5 Spd - got 15-16 MPG in town and 21-22 on highway --- with maybe a 23-24 on a long trip. My '03 4L V6 gets virtually the same but with 3x the performance.
