Decrease in mpg?
The Best V8 Stories One Small Block at Time
https://youtu.be/n-lWSaGMeik
https://youtu.be/TywucVlvnIw
I put on YouTube so anyone can chime in.
Edit; I also called the shop they stated that nothing in the tranny should have affected mpg, the guy was like change your fuel filter I'm like uhh I believe these are not serviceable? Meaning I have to take it to gm, or so I've read on the Monte Carlo forums. He also said he changed the intake gasket which I also believe I did about a year or so ago, the car does kind of sound like a bit of air is being sucked it but I didn't see a vacuum leak or anything indicating it was one when I visited my tuner last.
Maybe I should do a smoke test
From the sounds of your issue you need to have your tuner let us know what your fuel trims are coming out too. Also oem o2 sensor?






