355 rears?

Could've saved the cost of the rims and put LP24.5's on and it would've done the same thing. Or, drop $5-7K and a few days off and put some 4.17's in the rears. Tire swaps are free if you're needing tires anyway.

 

OK except the torque is being applied at the other end. eg the engine

 

I looked at low pro 24.5 but cannot get good traction tires in that size, only highway tires. I do a lot of gravel roads, farm yards and fields.
Another truck I had I did change the rears from 3.55 to 4.11 for $5,000.
I did need new tires so this was the way I went for now, but it seemed to be enough to do the trick,

 

More than 1 way to skin a cat...and you picked the best option for you. A great illustration for those thinking it only works the one way, as it shows you can go either way...as well as a cautionary tale about lowering your RPMs below the optimal range not being as efficient.

 

OK I thought about this and although I think your analogy is correct but is applied wrong,
While we have the truck in the shop jack up the truck. wheels off the ground we will make you Superman for a moment, you hold the torque wrench steady and I'll put the truck in gear to apply the torque. the higher numerically gears will apply more torque to the gage than the lower numerically gears ie 411 and 336 so the longer lever effect is the rear end ratio.
Now take the tires off it won't change the torque being applied.
The torque to the pavement doesn't change with tire size.

Lets try some math 1000 foot pounds of torque at the driveshaft x's 336= 3360 lb of torque vs 411 equals 4410 lbs of torque to the ground and tire size doesn't change that.

 

I have no problem with your results but I believe it's because your engine is in a 'better' spot.
IS it a generic truck or was it spec't for that job and if so by what or by who?

 

The torque applied at the axle shaft differs with the rear end gearing. The torque applied at the ground differs with the distance from the axle shaft. Larger tires = more distance = reduction in applied torque. Compensate for that with lower gears in the rears to increase the torque applied to the axle shaft.

The difference is slight, as we're only talking about a 1" change in the tire radius for each step. That is why it is only a slight change in gearing as well. 1-2 tenths. 3.55-3.36-3.23.

 

And searching for that "better spot" is exactly why someone would make an axle gearing or a tire size change...trying to put the engine into it's most efficient operating range for the speeds that person intends to run. If an engine is happiest running 1200-1500 RPMs, and you like to run 65 mph, you don't want it turning 1700 at 65. Likewise, if you spend much time at all on 55 mph roads, it'll drive you absolutely nuts if 55 mph is 1150 in your top gear and 1550 in the next gear down...constantly either lugging it or overrevving it. A slight gearing change solves that problem, putting you in the optimal range for the engine. That's all we're talking about here, is making sure the engine is going to be happy running with the gearing you're going to be running BEFORE dropping $5000-$8000 and missing a few days of work having the rear end gears swapped out...because if you screwed up the calculations, it'll cost you ANOTHER $5000-$8000 and another couple days off work to put it back or try again. A simple tire swap takes no extra time or money if done when tires are being replaced anyway, and lets you test the water so to speak BEFORE you commit to a major change.

 

Put that way i will agree. If you are operating out of the torque curve I will agree that a tire change could be beneficial.