Differential Gears vs Transfer Case Gears

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Btw, with the older 100 series trucks, you don't have to do the input shaft and idler gear, so it's only about $1000 for the underdrive and 3:1:1 gears. That would make this decision easier, but unfortunately I need the extra $600 shaft and gear.
Can you elaborate on this? I was about to pull the trigger on underdrive and low range gears recently but decided against it after I was told about the new input shaft and idler that put the cost over the price of diff gears. Wondered if an 80 t case could have compatible parts. I could rob the components off a $150 junkyard 80 t case and still do the tcase gears myself for less.
I have a 2000. Do I still need the input shaft and idler? If not I would do it in a heart beat.


FWIW i just put 35s on. Coming from stock size tires. It's got slower acceleration but I actually like the lower RPM on the highway. I felt the gearing was too low before. I would cruise at 75mph and be at nearly 3000 rpm. Now it's more like 2400 (but the speedo reads 68)

In your situation, you've already committed to installing lockers. Just do the gears with it. Your old gears are slowly wearing, increasing backlash and increasing the possibility for breakage especially as you start to wheel it with bigger tires.
 
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Ramathorn15

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Can you elaborate on this? I was about to pull the trigger on underdrive and low range gears recently but decided against it after I was told about the new input shaft and idler that put the cost over the price of diff gears. Wondered if an 80 t case could have compatible parts. I could rob the components off a $150 junkyard 80 t case and still do the tcase gears myself for less.
I have a 2000. Do I still need the input shaft and idler? If not I would do it in a heart beat.


FWIW i just put 35s on. Coming from stock size tires. It's got slower acceleration but I actually like the lower RPM on the highway. I felt the gearing was too low before. I would cruise at 75mph and be at nearly 3000 rpm. Now it's more like 2400 (but the speedo reads 68)
I'm not totally sure which 100s don't need the input shaft, or if all of them do. I've just read somewhere that possibly the older 100s don't. Logic would tell me it's the 98/99, but I can't verify that. I would email cruiser outfitters.
 

Ramathorn15

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Believe we met at HIH 2017, that was a great video you did for it.

On my 2000 I have 4:88s and the T-case low crawler gears. I do not have the underdrive T-case high gears. I run 295 tires.
I did the install on the 4:88s and crawler gears and I think T-case effort is about the same, if you have the equipment to pull the Tcase.
"They" say you can do it without pulling Tcase, but feel I did a better job having it out. Did a full check of Tcase and found some bearings I felt needed to be replaced along with some scoring on oil pump so replaced it also. It was so much fun, I did an install of low crawler gears in a friend's 80.

I have been thinking deeper on this and have a different take.
I am going to agree with hoser don't bother with the underdrive. Rather than requoting hoser fine spreadsheet, I will give you my seat of the pants thoughts.
With my 4 speed guessing the underdrive would be a little more beneficial to me. I am happy without it, my RPM is about 5% higher than stock, to me that is no big deal, and can't really hear that difference. Before the 4:88 it was about 6-7% lower than stock (295 tires). With the higher RPM it does a little less downshifting on Interstate hill climbs. Also don't think you can relate higher RPM to more gas consumption, getting about the same MPG.

Must say I love, love my Tcase crawler gears. Don't need lockers as much now.

Edit - New thinking
With your stock diff gearing and t-case crawler gears you get a lower crawl gearing than I do = good stuff.
If you are up for opening up everything then go with both gear sets in T-case and lockers in diffs.
There have been suggestions to go part time while you are in there, I had the option to do that and decided for my usage it was not worth it.

"With just lockers, I really only have to deal with backlash". I have only done a few, but intuitively I would think tolerances of components would bite you. assume you are doing new carrier bearings but maybe not?
I probably won't open both up unless it's to do the underdrive kit. Way I look at it, I'm either only opening up diffs, or I'm opening up diffs and tcase for the benefit of low range gear and underdrive.

As for the locker install, typically you only have to set backlash as your not going to be adjusting the pinion. You could probably get a little bit better pattern by adjusting the pinion, but usually backlash adjustment is all that's really needed to still be good.
 

hoser

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So let me break this down then.

Stock 4 speed with stock tires, final ER is 3.24
Stock 4 speed with 34's = 3.56 (You would need the combination of the gears to equal 3.56 in order for 34's to turn stock RPM numbers)
Stock 5 speed with stock tires = 2.94
Stock 5 speed with 34's= 3.23 (You would need the combination of the gears to equal 3.23 in order for 34's to turn stock RPM numbers)

4.88 equiped 4 speed with 34's = 3.56
4.88 equiped 5 speed with 34's = 3.84


So I'd still be substantially higher geared than a stock 4 speed on stock tires (3.24 vs 3.84) and would also be higher than a 4 speed with 4.88 and 34's. Does that make sense?

Wait this still isn't making sense to me lol. The ER # goes up as I add higher gears. The tires are also making that number go up. Shouldn't they be combating each other?
So, let's just use an online calculator instead....

"Original" = 4 sp, 31's @ 70mph. Top gear transmission ratio is .75 Engine, RPM 2446
"Variation 1" = 5sp, 34's @ 70mph. Top gear transmission ratio is .72 Engine, RPM 2430

Engine speed ~ pretty much the same for both scenarios.

Screen Shot 2020-05-28 at 9.52.55 PM.png

 
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I’ve driven for extended periods at 85-90 where rpm is 3k or higher. Cruises just fine and no buzziness with the 4 speed. When funds allow, im throwing a harrop in the front and going to 4.88s in the diffs. I’ll take all the help I can get in a heavy 4speed.
 
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