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04 Jeep Grand Cherokee poor mpg than EPA rating

I have a 2004 Jeep Grand Cherokee I6 4x4 that I just got. It has 128,000 miles on it and I always drive it in 2WD. I got the Jeep with a bad tranny, so I had new rebuild tranny. Ever since I got it, I been getting 12mpg no matter how I drive, and I am a very light foot person. Some people on other forum have suggested that it could be a oxygen sensor or try replacing the air filter. What should I do? FYI- I know Jeep GC don’t get great MPG, but I was expecting EPA rating of 14-19mpg city/highway.

Try changing the thermostat. I recommend changing every 5 years anyway.
Test the temperature sensors.
Check for dragging brakes.
Check wheel alignment, and of course tire inflation.

If the tranny you bought was rebuilt, consider going to a dealer to have the software checked out.