It would be additive, right? Add the gear ratios then multiply the engine torque at the rpm per the trans gear selected. Then you can compare wheel torque curves.
Let's assume we have two otherwise identical trucks with one 3.21 and one 3.92. They're both pulling the same 6000 lb TT at 65 mph on straight ground with no real wind. Tire size is 32 inches.
Here are the calculated final gear ratios for the trucks, as well as the calculated RPMs they'd be at depending on which gear they're in:
Notice the blue cells. At 65 mph, the 3.21 could be in 6th gear at 2191 RPMs, and the 3.92 in 7th gear would be at 2194 RPMs. (I picked the details in this scenario because I know my truck and my trailer and what gear it would be in at 65 mph on a flat road with little wind, but the argument holds regardless of what the variables are, as long as the variables affect both trucks equally.)
I don't know what the 5.7 hemi itself is putting out at those RPMs, but it doesn't matter because it would be the same for both trucks because both trucks are within 3 RPMs of eachother. Meaning, while the 3.21 is in 6th gear, and the 3.92 is in 7th, if they're both at the same MPH and both at the same RPMs, they'd both be putting the same torque at the wheels.
If the 5.7 is at the same RPMs in both trucks at the same speed, it must be putting out the same torque to the wheels in both trucks.
Ok so now both trucks hit a steep grade and can no longer maintain speed. The 3.21 drops to third before it finally revs high enough to produce enough torque from the hemi. What gear would the 3.92 most likely be in? Gear 4 is 4602 RPMs which only differs form the 3.21 by 86 RPMs. It's the most likely gear, because if it was in 3rd it would be redlining at 5725 and if it was in 5th it probably can't put down the required power at those rpms - if the 3.92 COULD be at 3505, why would the 3.21 be at 4688 when it can be in 4th gear at 3768 RPMs and still be putting more power down vs the 3.92 at 3505 RPMs?
So again, we know the 3.92 puts down more power in the same numerical gear number. But this is only HALF the story; the other half of the story is that the 3.21 doesn't HAVE to be in the same numerical gear. It can, and will be in one numerically lower gear at pretty much the same engine RPMs; and when the RPMs are the same and the final gear ratio is the same, then both trucks are putting the same torque to the wheel.