New Muffler = Lost MPG's
 

Well I went and did it. Put on a flowtech performance muffler onto the jeep and it seems to have dropped the mpg's by 2. Still learning the ropes of the scanguage and new driving techniques, but I swear that thing has done the opposite of what I wanted, I'll leave it on for now until I get more data and work through some other mods but come the spring I may put the old one back on for some accurate testing of both the muff's.
Frustrated!

More air through the engine will require more fuel. I would not be the least bit shocked that your mileage dropped.

There is a lot that manufactures can do to improve mileage, but there is a lot of development that goes into most stock air flow systems. Which is not to say that some mods may help but if you do performance mods ones that advertise more low end torque are the most likely to help. Even then though you will still need to drive it right.

Did you disconnect the battery when you swapped mufflers? You've changed the airflow through the system, so you need to reset the ECU and let the computer relearn!

The computer will learn very quickly and adjust without the need to reset the ECU.

Changing just the muffler should cause very little difference either horsepower or FE. I've done a lot exhaust changes on cars over the years and have tested these on chassis dynos. To get a muffler to make any discernible difference you to need to do more to the system in before the muffler. Also every car is different. I had a 2001 Mustang GT that a put a set of Flowmaster mufflers on. Nothing. Next I put on longtube headers, X pipe with no cats, and a new cat back exhaust system picked up 30 hp. Next added a CAI and a chip. Got another 14 hp. When I went on trips I would set the cruse at 80 mph and still knock down 27 mpg. Not for a car EPA rated 23 highway. The car was making 265 rwhp so that equates to about 312 hp at the crank. Added about 52 hp at the crank but increased FE......go figure.

P.S. Before anyone bashes me this was in my pre-eccomodder days;)

I'm not sure what year the OP has, but OBD-II computers ('96 and up) DO NOT reset by disconnecting the battery

The Jeep ones definitely do. I've cleared a CEL that way before, and the Jeep definitely runs differently for a bit after a battery disconnect until the PCM relearns.

As far as mpg, in many cases, it's a matter of "New Muffler = Heavier Foot".

Are you sure that your right foot hasn't gotten heavier with the new muffler? When I threw a cherry bomb on my car my mileage dropped because I couldn't resist the feeling of power that more boost sooner brought. After the novelty wore off my mileage improved. Less back pressure should improve mileage for the same power output at the crank unconditionally.

Thanks guys. yes I already reset the computer, and yes disconnecting the battery then connecting the two wires did reset the computer, I was told by my mechanic and have read of many other liberty owners doing the same, it also cleared the codes, temporarily which have been popping up on the check engine light. Like I said, I have alot of research and testing to get to the bottom of it but definitely seems to have dropped the mpg's and not due to heavy foot because I am driving slower now than I ever have since I got my new scanguage.
Going out now to cover up my grille and see if I can notice any effects. Foound some cheap leather type material at a surplus store (black) to do the job.

I just have to ask. You say you have code "popping up", do they have anything to do with the poor mileage? Have you checked them out?