My old company had '05 Century condos with Egr 14L Detroits set to 430/1550. 10spd with either 3.55 or 3.70 rears. We ran ON/NY/PA/MI/IL. pulling 70-80k gvw. They averaged 5.5-5.7mpg - not impressive. Our '09-'10 Maxxforces got 6.5 out of the box.
I'm in an 09 cascadia with the series 60 and a 13. I've been around 6 all winter letting it idle 24/7. Last tank was 6.8 now that the weather is starting to warm up. We run Midwest and southeast so some hills but nothing too significant