I agree. The dealership that I have purchased both of my Ram trucks from said they order mostly all of their 1500's with the 3.92's. Said they have a lot of people come from out of state to buy trucks from them just for that very reason. It wasn't a huge deal to me either way. I certainly wouldn't mind that extra 1-2 mpg with the 3.21's, and I see on here that they perform well, based on some of the trailers I see being pulled around on here. But I'm also not upset with having the 3.92's, these trucks sure scoot! But have nothing to compare it to, never drove one with 3.21.I think a lot of it depends on the dealer.
To add, I wouldn't rule out a dragging/sticking brake caliper. Lower speeds will net lower mpg as it is unless you can get to a speed that allows you to cruise in a higher gear. In town my 5.7 gets 10-12mpg but that's between 0 and 40mph on a regular basis with a light load in the bed (500lbs) and 3.55 gears. 3.92's should net you better mileage in town as it takes less throttle to get up to speed, and 25mph ain't all that fast.I have a 2015 Ram 1500 Rebel which comes with the 3.92 gears and have owned the truck for just over a year but just recently my mpg have dropped and have noticed I’m only getting like 5-6 mpg at around 10-25 mph and it doesn’t matter if I’m on the gas or not.