I darkened the background slightly to help beauty of this truck stand out. ld

This a great truck, and the Million Mile spec package will last forever... as a high horse power it is not bad on fuel mileage, but there were tradeoffs as high power versatility  was a focus on this spec, too -  As an explosives hauler, the owner of this truck wanted the ability to easily keep with traffic in nearly all conditions.  He can do that.  

** @ 40,000 miles  - fuel consumption details, silver/blue T600 Custom sleeper. 

This truck pretty much always runs at whatever the posted speed limit is.  Under normal operating conditions the fuel has been running:

very light load or bobtail :  9 mpg

light loads < 60k  : 8 mpg

moderate loads 60k - 70k :  6.9mpg-7.25mpg

heavy   80k : 5.8mpg - 6.2 mpg