But 1st is a mechanical disadvantage with the 3.21s, which is important while towing. And while cruising on the interstate, the 3.21s are either going to be at less RPM in 8th which will make it harder to cruise, or more RPM in 7th which would consume more fuel.Real strong argument to justify your purchase, but saying the 3.92 improves MPG everywhere but open interstate just isn't accurate.
Looking at the above final drive ratio chart, I'd probably choose the 3.21 as well. Running one gear lower there's a slight mechanical advantage over the 3.92 truck. I tow heavy and I'm in no hurry in first gear, so that's a non-issue. Never use OD gears when towing so another non-issue. 3.21 for the all-around win.
Bottom line is that it's like the age-old 10-speed bicycle analogy.... nobody wants to pedal uphill in the higher gears. It requires a lot more energy to do that. It's actually the lower gearing that gives ease to keeping the wheels rolling.
For fuel efficiency and performance I'll stick with the 3.92s.
Also, if you choose to run larger tires like me that plays into it as well. With my current tires, it's effectively like having a 3.65 axle gear.