I had enough trouble with emissions on my 6.7 so am considering a 5.9 automatic truck with 3.73's. I posted a request in the 5.9 section for experiences with 5.9 economy, but here is some detailed info on what I've used since my 6.7 was new.

I recorded every drop of fuel I put through my 6.7 since new - it used 1574.37 gallons in 25,634 miles which is 16.28 mpg. These are unloaded miles most of it highway cruising 60-65 mph, 60 miles per day to and from work. I've seen overhead readings above 22 and some long trips were hand calculated over 19 mpg, but it's the long haul that I wanted to capture. It is all in an Excel spreadsheet - no overhead readings here.

The truck has had the G30 update, a completely new emissions system with turbo, new sensors, etc. The only major change that impacted the economy was the AY flash update where it lost about 1 mpg, but it came back with a new turbo and new sensors.