What diff gears for 99 XJ with 4L engine, AT, and 30 vs 31 inch tires? - JeepForum.com

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post #1 of 31 Old 05-13-2013, 03:18 AM Thread Starter
Charley3
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What diff gears for 99 XJ with 4L engine, AT, and 30 vs 31 inch tires?

I have D30 front, C8.25 rear diffs.

I live in a semi mountainous area with level (or near level) freeway in valleys and steep freeway in mountains (with 7% grades common, and the occasional 14% grade).

Want to be able to cruise at 70 mph on level freeway and get decent gas mileage doing it. Also want to be able to pull moderate hills (4% or 5% grade) in 4th gear at 70 mph.

Also, need gearing low enough to back up easily on steep hills. i.e. - need low enough reverse. Not worried about 1st because it's lower than reverse. So need diff gears to work for that too.

What diff gears for:

30 x 9.5 R15 tires? Actual tire height manufacturer claims is 29.5". I'm thinking 3.73 diff gears?

31 x 9.5 R16 tires (245/75R16)? Actual tire height manufacturer claims is 30.5". I'm thinking 3.73 or 4.10 diff gears?

Are there gear choices between 3.73 and 4.10 ?


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post #2 of 31 Old 05-13-2013, 07:35 AM
SuperSam
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There are 3.90, they are just harder to find. If you do a lot of towing with the XJ I would go 4.10 with the 31's. I have 3.55 and 30's right now and I want more RPMs out of it.
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post #3 of 31 Old 05-13-2013, 08:22 AM
wilson1010
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Big waste of money swapping gears on an AW4. That little handle on the console will solve all of your problems.

03 Rubicon; 99 xj with too much stuff to list; Unimog 406 (gone)
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post #4 of 31 Old 05-13-2013, 12:26 PM Thread Starter
Charley3
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Quote:
Originally Posted by SuperSam View Post
There are 3.90, they are just harder to find. If you do a lot of towing with the XJ I would go 4.10 with the 31's. I have 3.55 and 30's right now and I want more RPMs out of it.
3.9 gears for Chrysler 8.25 rear diff? Sounds perfect for 31s (I don't tow).

I currently have 30s with 3.55 and it's fine on level, and near level, highway, but NOT very good on mountain highways.

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post #5 of 31 Old 05-13-2013, 12:28 PM Thread Starter
Charley3
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Quote:
Originally Posted by wilson1010 View Post
Big waste of money swapping gears on an AW4. That little handle on the console will solve all of your problems.
What handle? The shifter?

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post #6 of 31 Old 05-13-2013, 01:12 PM
foggybottombob
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I assume he is talking about some feature that varies the transmission shift points related to engine rpm. My 91 XJ has this feature as a rocker switch on the dash. I am not familiar with the 99 XJ. I have 3.90/3.92s and 31" tires and it is perfect. But you can't get 3.90 gears in high pinion Dana 30.
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post #7 of 31 Old 05-13-2013, 01:46 PM Thread Starter
Charley3
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Quote:
Originally Posted by foggybottombob View Post
I assume he is talking about some feature that varies the transmission shift points related to engine rpm. My 91 XJ has this feature as a rocker switch on the dash. I am not familiar with the 99 XJ. I have 3.90/3.92s and 31" tires and it is perfect. But you can't get 3.90 gears in high pinion Dana 30.
I don't have that dash feature. That was discontinued in early 90s.

Why do you say 3.90/3.92 ? Do you have different gears in front and rear?

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post #8 of 31 Old 05-13-2013, 02:07 PM
Kevin108
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3.55s and 31s are plenty comfortable. What's the deal?

My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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post #9 of 31 Old 05-13-2013, 02:17 PM Thread Starter
Charley3
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Quote:
Originally Posted by Kevin108 View Post
3.55s and 31s are plenty comfortable. What's the deal?
I live on flatland near sea level between Cascade Mountains, but have to drive through mountains passes to go on any signifigant road trip. Mt Hood, Mt St Helens, Mt Rainier, and thousands of unnamed hills that would each be called Mountains in the Midwest and Eastern USA.

In this area, a stock XJ with 30s and 3.55s struggles (and I haven't added skid plates yet!).

But I don't want to gear to low because we have one freeway that is (relatively) level that runs parallel to Columbia River between the mountains.

I'm thinking 3.73 with 30s would be ideal, or 3.90 would be ideal with 31s. I think 4.10 with 31s would be a bit to low geared on level freeway.

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post #10 of 31 Old 05-13-2013, 02:41 PM Thread Starter
Charley3
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Googling I found D30 gears 3.91 and C8.25 gears 3.90.

It concerns me that they are different.

But they are close to same.

Are they close enough to same?

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post #11 of 31 Old 05-13-2013, 05:12 PM
Kevin108
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Are you sure your transmission isn't slipping?

Within 1% on the gears you're fine.

My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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post #12 of 31 Old 05-13-2013, 07:09 PM Thread Starter
Charley3
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I am not aware of any transmission problems. How do I check?

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post #13 of 31 Old 05-13-2013, 09:07 PM
Kevin108
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Check the color and smell of your fluid. It's it pink or is it more of a rusty brown color? Is there any odor besides that of ATF?

My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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post #14 of 31 Old 05-14-2013, 12:20 AM Thread Starter
Charley3
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My AT fluid is new. Fluid and filter were changed 2K miles ago.

No problems were reported by shop that changed it.

It feels like it's shifting good.

I know it's common for guys in my area to run 4.10 gears with 31s, but I don't want to gear that low. 3.90 sounds better to me (for 31s).

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post #15 of 31 Old 05-15-2013, 08:35 PM
sAe23
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Here is one of several charts you can use to decide what gears to use:
http://www.superlift.com/library/gears.asp

And you do realize that the words "Cherokee" and "decent gas mileage" don't go together, right?
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