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Old 10-09-2010, 10:20 AM   #1
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Gear Ratio Help

I need help figuring the most optimal gear ratio for what I'm doing to my 2004 LJ. My LJ is a 3 speed automatic and has the stock Dana 30 in front and the Dana 44 in back. I am looking to run 35x12.5x16 mickey thompson mtz's and need to know what gear ratio to use. From the post's I've read some people have said 4.56 and others 4.88.

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Old 10-09-2010, 11:00 AM   #2
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Welcome to WF, glad to have you here!

Ok, your '04 LJ has the 42RLE automatic transmission which is actually a 4-speed. Its 4th gear is Overdrive.

And while 4.56 and 4.88 is an appropriate ratio for 35" tires with other transmissions including the 5-speed manual (4.88) and older 3-speed automatic (4.56), your particular transmission has such a high Overdrive ratio (which produces low highway rpms) that you need a minimum of 4.88. Even 4.88 is not quite enough gear for 35" tires, 5.13 or even 5.38 is a much better ratio as it will lug a bit with 4.88 on the highway with engine rpms being a bit too low.

Unfortunately, your front Dana 30 can only go as low as 4.88 so that is the gear you should go with. Don't even think about 4.56, you'd hate it.

After my last TJ was stolen, I bought a replacement with the same exact 42RLE transmission, 35" tires, and 4.88 gearing. I wish it had 5.38 and even the previous owner, a friend of mine, admitted he screwed up by not having gone to 5.38. Mine has a Dana 44 up front so it could have been regeared to 5.38 which would have been better.

So the bottom line is go with 4.88. While performance won't be as good as it would be with 5.13 or 5.38, it will be adequate. Not so with 4.56.

Enjoy the forum!

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Old 10-09-2010, 11:48 AM   #3
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x 2 on the 488's. That is what is in my LJ. It does fine on the highway. I have gotten 16 mpg out of it on a couple of trips.
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Old 10-09-2010, 02:34 PM   #4
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Thanks for your input! My first jeep was a 98 Sahara and I just paid to have everything done to it, so I didn't really get my jeep experience. A couple years after I did that i realized that I didn't build my jeep, someone else did, and I really didn't like that. I decided to sell the sahara when I found my LJ (my dream jeep) for a great price. Anyways, to make a long story short, I appreciate the input because although I've driven a jeep and been on trails quite a bit, I wanted to know how and why all the parts worked so I could build the jeep I wanted. You both have been a great help and I'll probably post some more stuff in the coming months and years.
Thanks again.
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Old 10-09-2010, 04:50 PM   #5
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Before you build the front D30, if your budget can handle it, see if you can find a Rubicon 44 someone is upgrading from. Sell the Rubi locker and throw in an ARB f/r so you can go to 5.13s or 5.38s. This way, if you want to go to 36/37" tires later on, you can. I drove on 37s for 2 yrs and never hurt anything with factory 44's in my '06 LJ Rubicon. I should have gone 5.13s at a minimum instead of 4.88s.

With the 4spd auto, 5.13s and 35s will be nice. You will also get better crawling w/o the 4:1 t/case.
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