I have my truck lifted 6" with 35" tires and 20"s. But running rich is definantly messing with my gas milage.

 

Yea I have 35's All terrains, 20 inch Pro comp Rims, but with the stock 3.73 ls rear. I also have a 6 inch pro comp lift. Before the lift I would usually see 15 mpg just driving and not really trying to baby it, and then 17 if I was in trying to get some good MPG.

So yea, I think I am going to order new O2 sensors, and then if that does not fix it, I suppose spark plug change.