Question about gear ratio in a TJ Rubicon automatic with Rubicrawler

cj cake

cj cake

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Stupid question time...

I just learned about the Rubicrawler unit from Advanced Adapters. I was wondering about how it could be used and wanted to ask a few questions here.

The Jeep I an driving is a stock 2006 TJ Rubicon with Auto Trans. I am going to eventually get it on 35" tires with a lift. I am currently running the stock 4:11 gears from the factory. I plan on very mild trails and rocks, nothing to crazy.

05-dusy-ershim-thompson-hill-1280x853.jpg
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I have read that the auto trans is an issue with the tall overdrive so I will need to re-gear to 5.13:1 gears. I have also read that those gears in the dana 44 have a small pinion and can get pretty hot. Also, the lower you go, the less tooth contact you have between the ring and pinion.

Now I was reading how the Rubicrawler will allow you to run a 2.72:1 reduction between the transmission and the transfer case. This got we wondering....

1. Could you use the 2.71:1 range in the Rubicrawler to increase engine RPM and run it with a high gear ratio in the differentials. I am talking about doing this on the roads, like at 55 mph. So as an example, running in 2HI in the T-case, low range (2.72) in the rubicrawler, and 4.11 gears on 35" tires?

2. If #1 is a stupid idea, then is there a way to change the overdrive in the transmission to make it less aggressive. Then say only need to run 4.88 gears in the differentials?

3. If the Jeep was re-geared to 5.13, and the Rubicrawler also installed, is the Low - Low ratio too low for 35" or even 37" tires. Assuming I am gentle on the Jeep.

Don't flame me too much. I am a rookie and just wonder how all this gearing is intended to be used.
 
You want 5.38 gears for 35s. Don't consider the RubiCrawler in the need to regear. The RubiCrawler is excessively low with the 241. You won't be putting 37s in these axles, so the gearing doesn't matter. No one has regeared the 42rle. We are stuck with what it is. Stop paying attention to old, incorrect gearing advice about small pinions.
 
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cj cake said:
Stupid question time...

I just learned about the Rubicrawler unit from Advanced Adapters. I was wondering about how it could be used and wanted to ask a few questions here.

The Jeep I an driving is a stock 2006 TJ Rubicon with Auto Trans. I am going to eventually get it on 35" tires with a lift. I am currently running the stock 4:11 gears from the factory. I plan on very mild trails and rocks, nothing to crazy.

View attachment 409668

View attachment 409669

I have read that the auto trans is an issue with the tall overdrive so I will need to re-gear to 5.13:1 gears. I have also read that those gears in the dana 44 have a small pinion and can get pretty hot. Also, the lower you go, the less tooth contact you have between the ring and pinion.

Now I was reading how the Rubicrawler will allow you to run a 2.72:1 reduction between the transmission and the transfer case. This got we wondering....

1. Could you use the 2.71:1 range in the Rubicrawler to increase engine RPM and run it with a high gear ratio in the differentials. I am talking about doing this on the roads, like at 55 mph. So as an example, running in 2HI in the T-case, low range (2.72) in the rubicrawler, and 4.11 gears on 35" tires?

2. If #1 is a stupid idea, then is there a way to change the overdrive in the transmission to make it less aggressive. Then say only need to run 4.88 gears in the differentials?

3. If the Jeep was re-geared to 5.13, and the Rubicrawler also installed, is the Low - Low ratio too low for 35" or even 37" tires. Assuming I am gentle on the Jeep.

Don't flame me too much. I am a rookie and just wonder how all this gearing is intended to be used.
Click to expand...

I have a 2006 auto with non rubicon transfer case and rubicrawler. I run 35” tires and 4:56 gears. You cannot run the rubicrawler in the highway to gear it lower, it’s too low. With both cases in low my Jeep will drive through the brakes but it works great on super steep hills up or down. With the rubicrawler you do have the option of 2nd low. I will use this on steep less technical trails. With your rubicon case and the rubicrawler you will have 2.72, 4.00, and 10.88 ration. The 10.88 is very low and I doubt you would use it.
 
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Shows what I know about "newer" vehicles with automatic transmissions. But however you do it, no OD will keep highway RPM from dropping too low.
 
cj cake said:
So as an example, running in 2HI in the T-case, low range (2.72) in the rubicrawler, and 4.11 gears on 35" tires?
Click to expand...
Not that it's really an option, the transfer case and/or Rubicrawler gearing is a multiplier. Your final ratio would be way lower than the recommended 5.38s.

cj cake said:
If the Jeep was re-geared to 5.13, and the Rubicrawler also installed, is the Low - Low ratio too low for 35" or even 37" tires. Assuming I am gentle on the Jeep.
Click to expand...
Some complain the 241 / 4:1 is already too low if you need wheel speed for sand or mud.
 
I have a 2006 LJ rubicon auto with a rubicrawler and 241. I ran 4.88’s with 37’s and it sucked on the highway. OD off all the time. I’d go 5.38 gears. You cant use low range at highway speeds.
 
For 35's and your 42RLE you need to regear to 5.38, not 5.13 which is not quite enough for 35's with the 42RLE. Gear your axles for the highway, not for your rocky trails, where 5.38 is ideal for 35's. I regeared this TJ from 4.88 to 5.38 and everything including its mpg improved significantly. Mpg went from 11 mpg to 14.6 after installing the 5.38 and the highway driving was a LOT more enjoyable too. And on the highway I consider 5.38 just adequate, I would not like the slightly lower highway rpms of 5.13 at all.

And with the Rubicon's 4:1 tcase and your automatic my strongest possible personal opinion says you don't need a Rubicrawler for your mild rock crawling like in your photo. My TJ Rubicon did lots of rock crawling on some pretty darned big rocks/extremely tough trails and it crawled beautifully with its 4:1 tcase and 42RLE automatic without a Rubicrawler. I could crawl at .01 mph if needed with that combination. :)
 
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jjvw said:
You want 5.38 gears for 35s. Don't consider the RubiCrawler in the need to regear. The RubiCrawler is excessively low with the 241. You won't be putting 37s in these axles, so the gearing doesn't matter. No one has regeared the 42rle. We are stuck with what it is. Stop paying attention to old, incorrect gearing advice about small pinions.
Click to expand...

Who do you recommend for the 5.38 gears? Can you point me in the right direction so I can read some newer correct, not "old, incorrect gearing advice"?
 
Jerry Bransford said:
For 35's and your 42RLE you need to regear to 5.38, not 5.13 which is not quite enough for 35's with the 42RLE. Gear your axles for the highway, not for your rocky trails, where 5.38 is ideal for 35's. I regeared this TJ from 4.88 to 5.38 and everything including its mpg improved significantly. Mpg went from 11 mpg to 14.6 after installing the 5.38 and the highway driving was a LOT more enjoyable too. And on the highway I consider 5.38 just adequate, I would not like the slightly lower highway rpms of 5.13 at all.

And with the Rubicon's 4:1 tcase and your automatic my strongest possible personal opinion says you don't need a Rubicrawler for your mild rock crawling like in your photo. My TJ Rubicon did lots of rock crawling on some pretty darned big rocks/extremely tough trails and it crawled beautifully with its 4:1 tcase and 42RLE automatic without a Rubicrawler. I could crawl at .01 mph if needed with that combination. :)
Click to expand...

I would think the nicest thing about adding a Rubicrawler to a Rubi would be getting the 2:72. 4:1 is often too low, especially on smaller tires, and 1:1 is sometimes too high.

The low-low 10 whatever:1 in a Rubi would be worthless. But put a Rubicrawler in front of a 231? Magic…
 
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Jerry Bransford said:
And with the Rubicon's 4:1 tcase and your automatic my strongest possible personal opinion says you don't need a Rubicrawler for your mild rock crawling like in your photo. My TJ Rubicon did lots of rock crawling on some pretty darned big rocks/extremely tough trails and it crawled beautifully with its 4:1 tcase and 42RLE automatic without a Rubicrawler. I could crawl at .01 mph if needed with that combination. :)
Click to expand...

Agreed!
 
NashvilleTJ said:
I would think the nicest thing about adding a Rubicrawler to a Rubi would be getting the 2:72. 4:1 is often too low, especially on smaller tires, and 1:1 is sometimes too high.

The low-low 10 whatever:1 in a Rubi would be worthless. But put a Rubicrawler in front of a 231? Magic…
Click to expand...

The 2:72 and 4:1 are great options. I use the 10:1 very rarely. Usually on super steep downhill for engine braking. Helps with control.

Jeep came with the rubicrawler. I would have never bought it myself but it’s nice to have.
 
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cj cake said:
Who do you recommend for the 5.38 gears? Can you point me in the right direction so I can read some newer correct, not "old, incorrect gearing advice"?
Click to expand...

My intention is to use Revolution Gear when 5.38s become available again.

As far as outdated thinking, the old fear was that these small pinion heads would twist off. That doesn't happen. One good reason why this doesn't happen is because with lower gear ratios, the mechanical advantage is in favor of the gears. Less effort is needed to spin them. You know this already from riding a multi gear bicycle. A second bit of old incorrect thinking is that many neglect to gear for the overdrive. Which means that any transmission with an OD would be geared too high following this method.
 
cj cake said:
1. Could you use the 2.71:1 range in the Rubicrawler to increase engine RPM and run it with a high gear ratio in the differentials. I am talking about doing this on the roads, like at 55 mph. So as an example, running in 2HI in the T-case, low range (2.72) in the rubicrawler, and 4.11 gears on 35" tires?
Click to expand...

If you ran the 2.71 range in the Rubicrawler on the highway with 4.11 gears on 35" tires, it would behave similarly to running 35" tires with 11.18:1 gears in your axles (2.71 * 4.11 = 11.1792). And that's assuming that it's okay using the Rubicrawler for long durations at high speeds. However, AA states that the Rubicrawler "cannot be used at engine speeds above 5000rpm when the transmission is in overdrive". According to gear calculators, with 35x12.50x15 tires (assuming 600 revs per mile), 4.11 gears, and a 2.72:1 underdrive, you would only be doing 64.8mph at 5000 RPM. 55mph would be around 4500 RPM, which is a bit wound up for highway cruising.

That is why it's not a good idea or a reasonable solution.
 
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NashvilleTJ said:
I would think the nicest thing about adding a Rubicrawler to a Rubi would be getting the 2:72. 4:1 is often too low, especially on smaller tires, and 1:1 is sometimes too high.

The low-low 10 whatever:1 in a Rubi would be worthless. But put a Rubicrawler in front of a 231? Magic…
Click to expand...

I second this. My original goal was to buy an 03-06 TJ with an auto and the 231, and add the Rubi Crawler. Would have been perfect with the 231.

Only reason I didn’t was because I found my current Rubicon local to me at a killer price. The 4:1 doesn’t justify adding one if you’re properly geared IMO.
 
deadbeat son said:
If you ran the 2.71 range in the Rubicrawler on the highway with 4.11 gears on 35" tires, it would behave similarly to running 35" tires with 11.18:1 gears in your axles (2.71 * 4.11 = 11.1792). And that's assuming that it's okay using the Rubicrawler for long durations at high speeds. However, AA states that the Rubicrawler "cannot be used at engine speeds above 5000rpm when the transmission is in overdrive". According to gear calculators, with 35x12.50x15 tires (assuming 600 revs per mile), 4.11 gears, and a 2.72:1 underdrive, you would only be doing 64.8mph at 5000 RPM. 55mph would be around 4500 RPM, which is a bit wound up for highway cruising.

That is why it's not a good idea or a reasonable solution.
Click to expand...

Thank you this reply. I understand that it was not meant for this purpose, but you make it clear WHY it is a terrible idea.
 
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cj cake said:
Thank you this reply. I understand that it was not meant for this purpose, but you make it clear WHY it is a terrible idea.
Click to expand...

You're welcome. People can tell you that something is a bad idea all day long, but sometimes it helps to understand why.

Jamison C said:
I second this. My original goal was to buy an 03-06 TJ with an auto and the 231, and add the Rubi Crawler. Would have been perfect with the 231.

Only reason I didn’t was because I found my current Rubicon local to me at a killer price. The 4:1 doesn’t justify adding one if you’re properly geared IMO.
Click to expand...

I'm confused by this. Why would you want a RubiCrawler for a 231 but not a 241OR? It seems like it would offer a lot more flexibility with the 241OR.

231 / RubiCrawler Ratios:
  • TC H / RC H - 1:1
  • TC H / RC L - 2.7:1
  • TC L / RC H - 2.7:1
  • TC L / RC L - 7.29:1
241OR / RubiCrawler Ratios:
  • TC H / RC H - 1:1
  • TC H / RC L - 2.7:1
  • TC L / RC H - 4:1
  • TC L / RC L - 10.84:1
Based on that, it seems like the 241 OR with the RC would be a really sweet setup!
 
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