High/low range Transfer case and differential gearing (1 Viewer)

[kevgentile]

Figured I post the topic for discussion since it's a bit spotty throughout the 100 series forum.
My specific interest is to acquire feedback from others that not only completed the 10% under drive in their
transfer case but also converted their ring and pinion to 4.56 or 4.88; specifically anyone with the a343f/4 speed trans.

I've completed the hi and low range gear swap into my t-case along with the part-time 4wd kit. i have found i still
have a bit of gear hunting due to the extreme climbs we have here out in the CO mountains. My thought was that being
able to stay in 4th/OD would be more advantageous vs turning off OD which kicks down to 3rd and really revs out the engine.

Thoughts comments, and personal experiences welcome.

 

[GTV]

What’s your tire size?
Carrying a lot of weight?

I did the transfer case gears and I’m very happy with it even though it is still slightly under geared compared to factory. But I also have a 5 speed, more power (‘07), my 34.5” wheels/tires are relatively light and I’m weight conscious overall. It drives very well, even through the mountains.

 

[kevgentile]

What’s your tire size?
Carrying a lot of weight?

I did the transfer case gears and I’m very happy with it even though it is still slightly under geared compared to factory. But I also have a 5 speed, more power (‘07), my 34.5” wheels/tires are relatively light and I’m weight conscious overall. It drives very well, even through the mountains.

Guess i should have mentioned that I'm on 35's.

The 10% under drive has been a decent improvement by itself, but was seriously thinking of changing out the
ring and pinion to 4.88's. I need to open the front anyways and thought of doing a locker and gears at the same time.

 

[GTV]

So you're still a bit under geared as well, probably by about 3% if your 35's are true to size. 4.88's will be a big jump, 13.5%. It will definitely get out of the hole faster and hold top gear easier but might be a bit buzzy at highway speeds. There are others running that combo but gearing is a very subjective matter that depends on a lot of factors.

 

[kevgentile]

So you're still a bit under geared as well, probably by about 3% if your 35's are true to size. 4.88's will be a big jump, 13.5%. It will definitely get out of the hole faster and hold top gear easier but might be a bit buzzy at highway speeds. There are others running that combo but gearing is a very subjective matter that depends on a lot of factors.

Holding 4th and not having to run near full throttle or turn-off O/D is what I'm after. The 3rd gear ratio is 1:1 where 4th is .75. I figured that the 4.88
gearing would allow me to keep the rpm's needed to climb these CO mountains comfortably at 60-65 mph while holding 4th.

 

[GTV]

I'd recommend playing around with a gearing calculator. 4.88's with the 10% under drive on 37's would actually be over geared. On 35's it will be fairly aggressive compared to stock.

 

[7toy9]

If he were to go to 4.88s and main downside is that at highway speeds his rpms are high (probably pushing 2.5k/3k at 70 mph), is the main concern the noise and high whine from the rpms? Or is running at that rpm putting too much stress on the transmission and engine?

 

[kevgentile]

I'd recommend playing around with a gearing calculator. 4.88's with the 10% under drive on 37's would actually be over geared. On 35's it will be fairly aggressive compared to stock

I have been thru all the gear/rpm calcs and the problem is that these can't take into account or simulate the load applied on steep grades.
The current 10% under drive with the stock 4.30's is actually very nice. However, I am finding that these 3-4 mile 5-6% climbs are still
causing a lot of stress due to having to run with O/D off which is dropping into 3rd (1:1 trans ratio). If there were just a few climbs and were short I'd not worry about it.

Heres what I came up with;

10% UD w/ 4.30 r/p (e/r = 4.73) , O/D OFF = 1:1 trans ratio equals ~ 2750 rpm @ 60 mph.

10% UD w/ 4.88 r/p (e/r = 5.30), O/D ON = .75:1 trans ratio equals ~ 2300 @ 60 mph ( with O/D off equals ~ 3000 rpm @ 60 mph)