So i have a RCSB silverado with a 4.8 and an open diff wrapped in 3.23 gears. the stock tires are 245/70/16 which are 29 inches high. (with no weight on them) i am currently running 265/70/17 (31.8 inches high with no weight on them.). I am going to be swapping to a eaton posi wrapped in 3.73's and a nelson tune. I was wondering if i would still be at about 20 MPG on the freeway because of my tire size and what kind of gains i can expect from the trio. I would imagine my city MPG would increase slightly?

P.S. Anyone have anything bad to say about a nelson tune? I cant find anything to deter me.

 

My 06 CC has 22" wheels, cam, tune, full exhaust and 3.73's. I get 19+ mpg on the highway at 70-75mph. A regular cab should get even better.

 

What kind of terrain do you live in? If it has hills and what not then I'm sure your mpg will stay around the same, if not improve. Extremely flat I don't see an improvement on just hwy mpg, but shouldn't notice a drop either.
Nelson is highly recommended around here.

 

With my RCSB 4.8 I didnt get anywhere close to 20mpg

 

Pshhhh, I don't think my saw 20 mpg before modding. Lol

 

When my truck was 100% stock (5.3), I got 420 miles per tank like clockwork. Every tank for over a year. I think that's pretty good, and that's with factory 3.73's and a 30.5" tire. I'm getting a valutrac in a couple of months and I'm still debating changing the 3.73's out for 4.10's. I just don't want to waste my money and not feel any difference. I think you'll be the same or better O.P.