I'm new to these forums. 3 weeks ago I bought a 07' 3500 crew cab dually 4 x 4 with the 5.7 ctd. The truck has 90k on the odometer with an automatic trans. The first tank of fuel I put thru it netted 13.2 mpg. I installed a K&N cold air kit for the next tank and the mileage went to 14.7. The next mod was a MADS Smarty Jr. programmer set to the performance mode, this thing makes stupid power and the mileage picked up to 15.2 in town and 18 on the highway. I have not changed the tune in the truck to check the mileage or the towing tunes yet. My question is how much would a cat back exhaust help the mileage? What would it hurt to remove the cat and do a full stainless steel exhaust? For a full exhaust would you run a 4 or a 5 inch system? I do a lot of towing ( 7000 # enclosed trailer ) and am looking for the most fuel mileage posssible without sacrificing the power.
Thanks

 

Just so we know, is it a:
5.7 Hemi
5.9 CTD
or 6.7 CTD?

 

Sorry, it is the 5.9 cummins turbo diesel.

 

I am by no stretch of the imagination a mechanic. I have spent a lot of time on the forums and think I've picked up a few answers to your questions...

1 - Exhaust - From what I've read... don't bother. Dodge comes with a 4" system that is fairly well designed.

2 - I also get ~18-20mpg on the highway with my truck. I have a retrax box cover, not sure if it has made a difference. I bought it for my 1500 when I had it. I run on economy / tow mode on my bullydog. If you are pulling 18 out of your dully, I'm thinkin' your are doing pretty good.