[Hammer]

Definitely like my 3.92 especially with the E-Rated tires which added some unsprung weight to move. I have a 2018 and the Gen 2 runs out of steam quick on hills so every gear matters it seems.

 

[Hammer]

basically the taller the gear ratio/final drive the more power you need to pull the same weight. it only becomes a problem when you introduce a resistance and as the resistance goes up the taller gear has to shift down. eventually the taller gear will run out of shifting down before the shorter gear. eventually the taller gear won't be able to move it and the shorter gear will be able to.

it's actually very simple
the 3.92 can pull more from a dead stop than the 3.21, so as you approach a resistance nearing a dead stop the short 3.92 gear wins. every single time all else equal.

so the 3.21 runs out of gear before the 3.92 with the same weight.

 

[Hammer]

fwiw I lost 2mpg with general at/x E rated 285/60/20 tires. drove in the rain yesterday and instead of skating around on garbage good years I was feeling confidence in the generals. they were pushing water our from under truck like a boat.

 

[Hammer]

I add this.
based on the comments from the oil viscosity email chain and the changes they made to the shift points during that flash update many years ago, the 3.21 HFE was a masterful bit of marketing that is (maybe) costing them engines lugging them around at low rpm high, low oil pressure scenarios where if you were to scar a bearing race that would be the time.... specifically the lower oil pressure (18 idling?)

heat burns fuel cleaner, generates less soot, keeps the EGR cleaner and that horrible hesitation off the line when cold is a shorter period of time because the truck heats up quicker turning more rpms at slower speeds

if for only those reasons, I'll call it engine survivability, I'd take the a 4.10 gear if they offered it... as it is, I wish I could make tow haul mode button shut tow haul off, instead of turn it on.

the 3,92 simply turns more rpms for the same speed and that seems ideal for longevity

 

[Hammer]

tire diameter with 60 vs 55 is 3.3% larger. you're speedo is off, and the actual distance you're going compared to what the truck says isn't accurate. that aside, you're basing the calculations off the odometer and not physical miles traveled so it's irrelevant most likely.

I got 29mpg with 3.92 gears on stock SRA tires in a Gen 2 2018 4x4 quad cab sport with factory air suspension. 34mpg in a Gen 3 should be easy to do.

to the topic at hand, the 3.92 can pull more because even at a steady state speed, the 3.21 or any lower gear requires more effort. On the premise both can move the weight, wind resistance and tire resistance are not ready state. there is an amount of resistance that at a given load, the engine cannot overcome it and maintain speed but with a shorter gear, you will have LESS of a loss. the shorter gear doesn't guarantee the engine can still pull it.

moving a force and keeping a force moving both require work for different reasons but the physics is the same to do it. the shorter gear all else equal can always pull more weight or maintain speed at a given resistance longer even if not indefinitely.

 

[Hammer]

I was suggesting earlier that the 3.92 gear has less propensity to lug the engine because it requires less work, all else equal to move the same weight. considering what FCA did at the time to do the same..... delay shifts and hold RPM's the only logical conclusion to take from that is 3.92 geared trucks should fail less.