New Muffler = Lost MPG's

[morpheous71]

Yeah I'm starting to doubt alot of the mods, I'm reading about on the jeep forums.
The O2 sensor that popping up is the front left (bank1, sensor1) apparently the real pain in the ass one to replace, priced the NTK's online for $76.24, so I'll at least replace this one even though my mileage seems to be getting better but temp's are up around here and my driving style is changing drastically. Maybe with the grille cover on and my KN Filter sucking air right from around the engine is helping, maybe. Got my best trailer mpg's today 21.8 behind the big truck, stayed solid jumping from 21.0 to 21.9 for about 20 miles on a mainly flat part of the 401 with speeds around 100KM. Yeah...
Going to go take some pic's and cover the lower grille tonight.