I occasionally used OD on the motorways (freeways), where the truck could be driven at reasonable rpm (around 75mph in OD due to 3.54 axle & 30" tires), but there was so much wind resistance that there was no fuel saving - in fact, the opposite.
In an aerodynamic car it would be a different situation.

My intention was to swap to a close-ratio T19, but the AOD died too quickly, so a C6 had to go in temporarily, followed by the NP435 swap later - T19 next week, Gary! Seriously!

In the process of all this though, I learned a couple of things (just 'seat-of-the-pants' observations, in my application)

- the power loss difference between a C6 & an AOD seems pretty much non-existent
- the C6 is actually slightly lighter than the AOD......10-15lb IIRC
- NOT using OD, the C6 & AOD gave me the same mpg, despite the AOD lockup in 3rd (& 4th)
-swapping from C6 to 4spd manual has so far gained 1mpg, running light (not towing or loaded) but I think the C6 mileage would fall behind, loaded.
-while the mpg difference between C6/Np435 is only slight, the power difference is noticeable, especially on initial acceleration & climbing a hill
- with the manual trans, my vacuum gauge reading was often 3 inches Hg higher on hills than with either automatic; but not much difference on flat roads.

Hope this might help someone