I got a muffler like this one back around August '11, and after I got it put on, I noticed my mpg's go waay down:

That is probably the exact muffler I have ^

I was thinking about getting this Flowmaster system here, I thought it would help me out with my mpg's but if it won't, let me know so I don't go wasting $300 on it:
Link to Flowmaster exhaust I'm looking at

 

It's all dependent on flow rate. Flowmaster should be able to tell you what that rate is.
Keep your head down.

 

Good morning recruit.
I can't think of any reason a muffler would cause a decrease in fuel economy aside from causing an exhaust restriction.
That shouldn't be the case considering that thing you installed is almost certainly a straight through design.
Maybe somebody stuffed bananas in your tailpipe.

 

stop stomping on the gas so much to hear your exhaust!

 

^ that sounds like the problem to me also!

 

lol you're probably right

 

its an age old problem, if you don't have an intake yet...just wait you will do it all over again then!

 

i got an intake first lol