So I tried a little experiment - I reset the truck computer at the bottom of a large hill (Heading to taco bell, of course. Why they can't build one at the BOTTOM of the hill...) and reset it again at the top.
Here's the pictures from the truck's dash board - Yes, the 6th gear picture is CLIMBING the hill and the 3rd gear picture is going DOWN the hill. The transmission automatically downshifts to increase engine braking (And I was still using the brakes).
Heading up hill -
Heading down hill - I had to stop at every blasted stop light on the way down the hill. Magnetic sensors only. Did it again today and the same stretch actually ticked the computerized counter up over the 40 MPG mark but it's pitch black and snowing so no picture.