Pretty much anything I do has one motive: It amuses me to do so.
I'm not just trolling, though, thats not amusing. I think perhaps you've exaggerated my position a bit.
In bullet points, my fuel economy gospel is:
1) I'm sure 3.21s do better at fuel economy. It's why malaise era cars had 2.56s and sucked.
2) I don't think it's 10% difference, or 2mpg, for most drivers.
3) The 3.92s will accelerate faster, both due to the mechanical advantage out of the gate and because of the speeds at which the truck must shift. The crankshaft doesn't go from 0-redline instantly, and getting off the line quicker and spooling the crankshaft faster equates to faster acceleration.
4) Unspoken, but 3.92s give you an option for upsizing on the tires with less headache if you should choose to for offroad performance or brodozer aesthetics.
5) For me, I've lost more money in the last 3 months of market downturn (paper losses, anyway) then I'll spend in gas for the next 20 years or more. I'm not sweating that, so I'm surely not sweating the difference in feeding a 3.92 truck vs a 3.21 truck. I understand different people are in different places economically speaking.