So far the 6.7 and 6-speed transmission has been a great engine/transmission combination for us. I know in my truck alone, I'm getting about 20% better fuel economy than my 2010 F450 at 13-14 mpg (both had 4.10 gearing and gross around 11k lbs on a daily basis). My dad's F350 gets about 16-18 mpg (the F350 has the 3.55 gearing). Our F550 with the 4.88 gearing gets about 8-9 mpg and it grosses around the same weight as my F450 on a daily basis. The mpg on the F550 is the only anomaly to me, but I'm attributing the big difference in mpg between the 450 and 550 to the deeper gearing of the 550.