my04 cummins has stock exhaust s@b cold air intake. is putting 5" exhaust worth or necessary to help mileage or is it just a for looks. i'm thinking about a smarty jr to help mileage would i need to do any tranny work for this since tranny is stock. any help would be greatly appreciated:doh:

I know I'm not a 3rd Gen but even my 2nd Gen with 3" exhaust that is straight piped I still pulling good MPG's.

Speed is the biggest killer of MPG's (55-60 MPH is best)

Tires is the second biggest killer of MPG's (Smaller are better)

my04 cummins has stock exhaust s@b cold air intake. is putting 5" exhaust worth or necessary to help mileage or is it just a for looks. i'm thinking about a smarty jr to help mileage would i need to do any tranny work for this since tranny is stock. any help would be greatly appreciated:doh:

An '04, 305hp/555torque........or and '04.5, 325hp/600torque???? My Bud has an '03 with 300k + miles on her. The '03-'04's don't have kittys, so when his exhaust went, he went with a 5 inch system with a straight thru muffler. It's loud!!! Not as loud as my '99 with the 3" straight pipe, but it's loud!! You're gaining nothing by going to the larger size. If you insist, go to a straight thru muffler. Most of the restriction is in the mufflers on our trucks.............even the ones with kittys!!!

I agree with the above statements, your not going to gain anything other than maybe a slightly lower EGT and thats debatable. The 04.5 already have 4" piping, the 03-04.5 are 3.5" either way you could just straight pipe it and get a decent sound without spending the extra cash. On the flip side of that, I love listening to a good sounding truck and I purchased a 5" for mine just because of that reason. Its all about personal preference. I am not sure how promotion of vendors goes on this site, but shoot me a pm if interested I will tell you who has the best deals right now.

My 06 had the 4" stock and I went with the 5". I have a flow through resonator and I can say my exhaust temp dropped, not a lot but did. (I don't have an EGT added to my truck, but borrowed a mobile device from where I retired from and did a test pre and post change out.) With the 4" you have and a flow through muffler or resonator, you should be happy. Myself, if I could do it again, I just would have removed the muffler and added a flow thru resonator, staying with the 4" piping. Slight increase in sound, but not bad at all.My friend has the 4" stock system and went straight thru by removing the muffler and inserting a straight pipe, and it was to loud. (His test.) He later added a resonator.

My truck was stock when the OEM steel system rusted out. I bought SS to stop from replacing it again. The OEM is 3" (in 01, my year) but smaller out of the muffler. 3" full length is an upgrade. The smallest SS was 4" full length with a straight through "muffler". It was too loud towing (as was my old built 454, headers & glass packs.) I added a FTE spiral resonator which quieted it down a lot. I did not see any MPG improvement. If I was going to do it again... skip the straight through muffler to save the cost... install the resonator as forward as possible.