Someone started a thread w/ this almost exact title for the 6.7's. Of course I might never get a straight answer. But who cares, I'd like to know anyway. I'm still dreaming of getting a Smarty Jr. someday in the future.

 

I get ~ 1 MPG better with my Jr.
The real difference is in how it helps with towing. G

 

I agree with Gordon, the best way to gain mpg: light foot. I was expecting more from my Jr. Then I set it back to stock and knew it was a wise investment. I keep mine at SW1 and it drives like a dream.

 

I got a Jr in December and had a 300 mile road trip to test it out that weekend. On the way there I had my truck stock and set the cruise at 70 mph. 15.2mpg on paper. On the way back same speed 18.9mpg. Temps were the same and both ways were very light traffic, interstate the whole way too. This was in my 05 4x4 with 3.73s and 33" Terra Grapplers.
Chris

 

I get 1.5 to 2 increase (hand calculated)

 

No increase at all...but, I didn't loose any either and gained 100HP...so that works for me

 

I saw a 2 MPG increase with my 2005 5.9 on SW3

 

I'm looking forward in buying a 5.9 (05-07) to get better fuel mileage. I don't really care about the horsepower or towing ability. I just want to get better fuel mileage. How can I get better mpg? Would it help to get a chip or programmer? If so, how much better would it be? And which chips and programmers are the best for fuel mileage?

 

CheTummins15, Everything really works together to give you those added miles. A lift or bigger tires obviously hinder that goal. Air and fuel help and a tuner maximizes the efficiency of your hardware. A cold air intake, manifold, lift pump (such as FASS or Airdog), and a larger exhaust (not stacks since they have a 90 degree bend) are basic add ons that will help with this. As far as tuners go, Ive run several different ones. A lot of them have economy settings, but Id say many are better at producing power than economy. The best Ive run so far has been the Smarty. If your truck is mostly stock, Id go with the Smarty Jr. I bought that tuner and saw 2-3 mpg gain at SW3 when driving nice. If you are going to run it hard, you wont see the gains you want and Id definitely tune it down to SW2 or maybe even SW1 to protect the tranny.