This is a snapshot of what I see on most road trips/highway travel in the PW when traveling at 70mph (speed GPS and mathematically corrected for tire size to 63mph). I have 37x13.50x17's which are 11.68% larger in diameter than the factory rubber. This equates to a -10.46% speed variance. If you take the 14.3mpg average seen in the pic (it's usually more like 14.7-14.9mpg) and multiply it by .1046 you get an adjusted 15.79mpg's (16.46mpg if you use the 14.9mpg average I see more consistently).
In town I see roughly 11mpg on average, corrected that's actually 12.15mpg, not the best, but no worse than the 4Runner and Land Cruiser on 33's we had a few years back. The range estimate shown is based off of the previous tank where I averaged under 10mpg's due to the majority of my travel being under 15mph off-road at the ranch.
Obviously there are other factors to consider like the increased rotational mass from the heavier wheel/tire combo, increased drag from the larger contact patch, and reduced aerodynamics from the higher vehicle and increased air flow underneath. The lie-o-meter is not always consistent when compared to real world calculations and I have no idea how far off the computer is on these trucks, but I will be sure to hand calculate the next interstate travel tank with the -10.46% variance for comparison. On a recent trip back from the coast I tried setting the cruise to a shown 57mph which is actually 63mph when corrected for the larger tire size. I averaged a shown 15.7mpg's on that 60 mile run which is 17.3mpg's when corrected, a small improvement but 1-2mpg's more for a 5-7mph reduction in speed isn't half bad if I have the time to go slower.
I don't know about y'all, but that is more than acceptable milage in my book for a 7000lb truck on 37's. How does your PW/Cummins compare?