Absolutely, but that's not what matters here. Because unless the driver is purposely driving the trucks differently, the 3.21 will always get you better MPG. If you driving your truck get 19 mpg on the highway, well you would get higher than that driving my truck.
So when a guys talks about prioritizing MPG, he's still going to "miss" getting better MPG by buying a 3.92.
If the owner has a heavy foot, going to a 3.92 means he might get only 17 mpg instead of 20.
If the owner has a light foot, going to a 3.92 means he might get only 20 mpg instead of 23.
That is the context here, a guy wanting to prioritize MPG while occasionally towing a light trailer. I get that a lot of you guys here are enthusiasts and don't care as much about MPG, but when somebody specifically lays it out so black and white like the original post, I would never ever recommend the 3.92 over the 3.21 for his usage. The 3.21 is built for this exact scenario. To me, recommending the 3.92 here is just as wrong as recommending the 3.21 to someone like you who wants to run bigger tires. Each rear end has advantages and disadvantages.
Even though that is the case, you can’t just be dismissive about recommending the other gear ratio because people do have the tendency to change their mind on something later on down the road so that’s why I, and a few other’s recommendations for the 3.92’s is important. Gives the guy something to think about for if he ever does decide to put on 35’s, or if all the sudden he decides to get a travel trailer in the future, he might be glad he went with the lower gears or he might be kicking himself later on for getting the higher ones.
Nobody knows, that’s why in my opinion it’s better to be prepared rather than realizing you made a mistake later on, like that guy I mentioned that I talked to on Facebook who was pissed because I guess he just didn’t realize that 35’s on 3.21’s was gonna screw himself up so bad.
You can still get good mileage on the lower gears. Even though I have some of the best tires for mileage in my Michelin Defender LTX’s that are stock size, a good tonneau cover for lessening drag, the active grille shutters still in place and the aero-mode on my air suspension, I’m probably still not a good candidate for sharing my mileage because I do have a heavy right foot, a very badass-sounding exhaust and a $600 radar detector that does a damn good job sniffing out the Smokies.
But in response to your comment about LT tires, I also never said that those would increase your payload. That was you who assumed that, as well. I only brought up the subject of LT tires because yeah, when you are towing a 10k+ thousand-pound trailer, I can kinda see how some people would be ok with sacrificing their ride quality with the thicker rubber coupled with 65 to 80 psi depending on if they were 8-ply or 10-ply just to increase stability and as far as I’m concerned, if you’re only staying around 8,000, even your factory tires aired to the max of 44psi should be fine so those people who run 8 ply or 10 ply tires are probably giving a **** less about their payload and are probably doing just fine despite the limitations of payload.
And one last time, since it’s all about safety these days and limiting and protecting against lawsuits, I would not be surprised at all if Ram actually sets our GVWR’s on the lighter side from what it actually could be. If I ever had the time come up to do it, I could probably even prove that to be true being that since my air suspension actually only tries to level the back end out to certain point before it quits putting air in the rear bags, I could load the back of my truck to that point and then go weigh it to see if it stopped leveling at my payload rating or if it actually went beyond that because see, back when I pulled my buddy’s mustang to the tranny shop, the back end of my truck did not squat at all; and this was done with a heavy duty steel flatbed trailer with built in heavy duty ramps with a 2-5/16th ball hitch and 10-ply tires on it.
I didn’t have my fancy WeighSafe drop hitch yet with the built in tongue weight scale so I was using my regular ole straight hitch from Walmart that had all the ball sizes on it but even though I have no idea what that trailer weighed or how much his car weighed, we had it loaded to where his front tires was about dead center between the front of the trailer and the front of the trailer fenders so if my payload was exceeded, I have no idea. It was just me and my girlfriend in the front of the truck with my older tonneau cover on the back, the normal tool box and plastic tote I carry around that has a bunch of other “trunk” items in it, my plastic ramps to pull my back tires on to lower the back of the trailer enough so he didn’t rip his front clip off and about a 2x12x10 board’s worth of wooden blocks to help in loading his car. I really wish I would’ve weighed but but I never thought to do it.
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...... oh SNAPS ! 
