Go back to the 87 tune, run 89 octane gas, and don't worry bout the stat. A fair bit of your mileage drop may simply be the switch to winter blend gas...... or, you may have an O2 sensor that is lying to the PCM. In which case, your data logging will show everything working as it should. Pay attention to long, and short term fuel trim. Should be pretty small percentage. If you break into double digits, first thing I would look at is O2's.

 

Before any changes , I think I would also check to make sure it's not something dragging , like a brake caliper or a dry u-joint .

 

Agree with onegoodmason, check for that kinda stuff first, all the free checks. I havent seen it asked yet, when was the last time you did a full tune-up? Cap, rotor, plugs, wires all those goodies. I would def swap t-stats for a stock 195 again and go back to the 87 tune. Good heat IMO is a necessity for winter. How old is the front o2? Might wanna replace it if its been in there for a long time.

edit: missed you had new o2's .

 

Happy Hollidays! and Merry Christmas!

This helped, Thanks! , but why?

Last week about, 70 miles from half to gas light coming on. I switched to the 87 tune, no 89 here so I ran 91. Yesterday and today's driving 120 miles from half to gas light coming on. All under pretty similar driving conditions.

Edit: On another note, I assumed I could use the data logging with my tuner and vehicle. Seems data logging does not work with this model year

 

Engine is running the way it is supposed to, octane a bit higher than necessary, so you get very little, if any, spark knock, (decreases power) everyone is happy. For the most part.

Data logging is a product of the scanner you use, if your PCM is willing to cough up the data, scanner should be able to record it. Especially on an 01......