I have 06 2500 cummins anybody know which way is better for fuel milage. full flow muffler with cat or without? thanks

 

you want back pressure in your exhaust system in my opinion... if you open it up with a straight thru muffler & no cat, you'll loose mpg but gain upper end hp.

 

I believe it depends on your driving style and commute. If you do a lot of highway driving, low back pressure is better. If you do stop and go it is better to have some back pressure. Of the two, I say straight pipe is better. I am addicted to the sound