All else being identical, except rear gear, here’s what you’d be at RPM wise assuming 265/70r17 tires. 3.21 on the left vs 3.92 on the right.
View attachment 174471
Another interesting thing is that the 8 speeds final drive ratios are so wildly different than the older 545RFE/65RE, the 8 speed being better obviously. 3.21+8 speed on left, 3.92+5 speed on right.
View attachment 174473
Those numbers look a little off compared to the 8hp75 I'm running, I'm not sure what transmission he's running though so that may be it.
But in any case, the trick here is to remember that you won't be using the same transmission gear in both trucks (3.21/3.92) at the same time. If you compare 6th vs 6th in both trucks at the same mph, yes the 3.92 puts down more torque at the wheel. However at that speed the RPMs would be significantly higher in the 3.92 (if the 3.21 can pull at say 2100 rpms, so can the 3.92), and so the transmission would likely upshift to 7th gear putting the RPMs and torque back down to the same level as the 3.21 in 6th. Therefore the torque advantage of the 3.92 just got canceled by the upshift.
I use the example of buying a dozen eggs from a vendor; you have two choices:
A) 4 eggs per carton, 3 cartons purchased = 12 eggs
B) 3 eggs per carton, 4 cartons purchased = 12 eggs.
In other words, 4 * 3 = 12 just like 3 * 4 = 12, doesn't matter what order you run those numbers, you still have a final multiplication value of 12.
That's what's happening here with transmission and gear ratios. You can increase the gear in either one to get you to the same point.
The only place where that doesn't work, is when both trucks are in first (3.92 has pulling advantage) or when both trucks are in 8th (3.21 gets better mpg and lower rpms) because at that point there is no longer any room for the transmission to make up the difference in the rear axle ratio.
