Differential Gears vs Transfer Case Gears

Ramathorn15

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Hey mud!

I figured this could be an interesting discussion. I recently picked up arb lockers during the memorial day 10% sales. I originally really wanted the harrops, but when it came down to it, I couldn't justify $2700 over $1800 for the Arbs.

Naturally, with locker installs in my near future, I began thinking about gears. I initially was just going to bite the bullet and go with 4.88 and be done with it. Then I started remembering that a few cruiser companies carry transfer case gears and I remembered them being much cheaper. The 10% underdrive gear gives the factory 4.10 the equivalent final ratio of 4.56 which sounds more on par with my 2006 vvti with 34's. I often cruise at 80mph so 4.88 I fear would be too low. I also remebered that you can do a 3:1:1 low gear in the tcase which is about 25% lower than stock 2:54(I think that's the factory ratio).

Anyhow, after investigating further, I learned that newer 100 series trucks have to replace the input shaft and idler gear with the underdrive kit which made it significantly more expensive. So I started breaking down pros and cons:

Tcase gears (~$1600)

Pros
1. Get lower low range (3:1:1)
2. 4:56 seems like it would be a better match for my truck

Cons
1. Probably more difficult to install. Looks to be about a 7-8 hour job
2. About $100 more expensive
3. People mention gear whine, but most installs so far seem to be in 80's which probably aren't as noise insulated as my LX


Diff Gears (~$1500):

Pros
1. About $100 cheaper
2. Already will have diff out for locker install so would be easier to install

Cons:
1. 4.88 may make my highway speed too high rpm
2. No lowered low range ratio (I know low will feel lower, but not the same)
3. I've always hated setting pinion preload, depth, and getting a good pattern. Not difficult, just can be frustrating putting it together and taking it apart 10 times. With just lockers, I really only have to deal with backlash which is easypeasy

Now to you mud, what are your thoughts? I'm leaning tcase gears because the added benefit of the 3:1:1 seems pretty substantial for only really $100 more. The install will suck though. Any pros or cons I'm forgetting?
 

Ramathorn15

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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.
 

guardcompany

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I am in the exact same situation and I'm excited to see the community feedback. Thanks for posting this... also... You are already making a mount I need at the exact time I need it, are you in my head? ha
 

hoser

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In your case, I'd choose the 4.88's. Follow me.... Take a look at the chart below for the A343F, blue column. Top gear on a stock 98-02 shows "3.24" effective ratio and this is for 31" tires. 34's are about 10% larger than stock and so you take the 3.24 x 1.1 = 3.56 to get back to stock ratio.

Okay, now look at the A750F, 4.88 column... ratio is "3.49" which is rather close to the 3.56's

So, if you've ever driven a stock 4sp 100 on the highway... it isn't buzzy.

new_gear_chart.jpg


Furthermore, regarding low range.... 4.88 / 4.1 = 1.19 or 19%.... so your low range will also be 19% lower versus the 25% for the t-case gears. Now, if you threw in the desire to do a part-time conversion in the mix... then T-case gears might be the way to go.
 

Ramathorn15

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In your case, I'd choose the 4.88's. Follow me.... Take a look at the chart below for the A343F, blue column. Top gear on a stock 98-02 shows "3.24" effective ratio and this is for 31" tires. 34's are about 10% larger than stock and so you take the 3.24 x 1.1 = 3.56 to get back to stock ratio.

Okay, now look at the A750F, 4.88 column... ratio is "3.49" which is rather close to the 3.56's

So, if you've ever driven a stock 4sp 100 on the highway... it isn't buzzy.

new_gear_chart.jpg


Furthermore, regarding low range.... 4.88 / 4.1 = 1.19 or 19%.... so your low range will also be 19% lower versus the 25% for the t-case gears. Now, if you threw in the desire to do a part-time conversion in the mix... then T-case gears might be the way to go.
I'm trying to follow this, but I think the 10% would be subtracted from the effective ratio, no? Bigger tires would reduce the final drive instead of increase it?
 

hoser

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In the chart, "effective ratio" happens before any tire calculations. So to upsize the tires 10%, you'd have to upsize the gearing 10%. 31" equals "1" and "34 inch" equals 1.1
 

Ramathorn15

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In the chart, "effective ratio" happens before any tire calculations. So to upsize the tires 10%, you'd have to upsize the gearing 10%. 31" equals "1" and "34 inch" equals 1.1
I get that. But with that thought process, if I add huge tires, let's say 2x factory (62") then my final effective ratio in 5th is closer to 3rd gear, which makes me think it should be subtracted as larger tires give you a higher final drive instead of a lower. Maybe I'm thinking about this backward.
 

hoser

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That's right, if you went to 62" tires....(multiplication factor of 2X), you could keep it in 3rd gear (on the freeway) and the engine would turn similar RPM's than stock tires in 5th gear.
 

nukegoat

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I considered this, but if I'm already in there, I might as well give my driveline a bit of tire compensation.
Ok, then diffs. The underdrive gears are inherently wonky engineering unless someone can prove otherwise. A given design and gear pitch on a fixed axis can't just magically add or remove teeth without going to the boundaries (or beyond) of the intended tooth mesh. Have any broke? Not that I know of, but people report the whining
 

Ramathorn15

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That's right, if you went to 62" tires....(multiplication factor of 2X), you could keep it in 3rd gear (on the freeway) and the engine would turn similar RPM's than stock tires in 5th gear.
Ah. Okay. Now I'm with you. Thanks for leading me down the path lol.
 

Ramathorn15

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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
Stock 5 speed with stock tires = 2.94
Stock 5 speed with 34's= 3.23

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?
 

abuck99

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If youre going to dive into the TC, add the part time kit to your equation.

Heres the thing, youre already gutting the diffs to add lockers- ring gear comes off to swap over to the arb carrier, so theres really not much added labor(yes pinion swap) to install 4.88’s just the cost of gears and install kit. From a labor perspective seems to me regearing to 4.88 would be less work.
 
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...
Now to you mud, what are your thoughts? I'm leaning tcase gears because the added benefit of the 3:1:1 seems pretty substantial for only really $100 more. The install will suck though. Any pros or cons I'm forgetting?
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?
 
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Ramathorn15

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If youre going to dive into the TC, add the part time kit to your equation.

Heres the thing, youre already gutting the diffs to add lockers- ring gear comes off to swap over to the arb carrier, so theres really not much added labor(yes pinion swap) to install 4.88’s just the cost of gears and install kit. From a labor perspective seems to me regearing to 4.88 would be less work.
Yeah, very valid point. Only thing I worry about is being too high strung cruising at 80. Seems most report good results with 34's on the 4 speed, but with the 5 speed I've seen people mention 70-75 being a sweet spot, but I cruise at 80-85 typically (75mph speed limit here)
 

Ramathorn15

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I should add that I'm building a ~3500# trailer I plan to drag around the mountains. That's really the thinking around gears. I don't feel like I need them loaded up now, but I suspect that will change once I start towing.
 

abuck99

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Yeah, very valid point. Only thing I worry about is being too high strung cruising at 80. Seems most report good results with 34's on the 4 speed, but with the 5 speed I've seen people mention 70-75 being a sweet spot, but I cruise at 80-85 typically (75mph speed limit here)
Id think the 5spd OD ratio would yield lower rpms at 80mph vs the 4speed right? You’ll be fine with 4.56 but there will be a point where you regret not having the little bit extra gearing when you inch up to 35’s and your loaded down climbing mountain passes. Id lean towards 4.88’s.
 
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