XJ: gearing ?'s

[bails85]

so i believe i have a dana 35rear and a dana 30 front but i jst put on some 32s and am really under geared. my ? is what gears should i go with and do i have to just change the ring and pinion or is there other stuff i have to do as well. and also i need some wrenching info cuz i would like to do it myself. i keep seeing sizes like 7.2 and 8.5 what do those numbers mean? also what is the difference between spicer and dana because at the parts house all my brake stuff is dana but it says spicer on my u-joints and drivershafts. i just want some axle schooling. thanks.

[OU812]

Your cherokee probably has 3.07:1 gears. You should be OK with 32s and many people run 33s with 3.07 gears(but they don't enjoy it. Re-gearing axles takes some time, patients, specialty tools(~$300), and lots of know how. Its an arduous task to say the least. So before you think about re-gearing you need to think seriously about what you want out of your XJ. Do you want to stick with 32s? Or maybe you will want 33s or 35s? If so you should probably think about upgrading at least your Dana 35 in the rear. You don't want to have to re-gear twice if you don't have to.

IMHO

29-31 tires= 3.07 gears
33s=4.10 gears
35s=4.88 gears

[2blk2drs]

What year is your rig?

If its a MT (BA-10, AX-15) with a 4.0 you have 3.01:1 gears

If its an AT (AW4) with a 4.0 you have 3.55:1 gears

Dont Bother upgrading your D35, XJ axles are way to easy to come by.


If you want to regear on the cheap, and with the least amount of special stuff, go find a MT 4 banger. Those come with 4.10:1 axles and if you pull em yourself you mught get out of there for under $250 for both axles. (Hint when they ask you what it came off of the correct answer is 'station wagon' its technically true, and they charge about 3/4 the price for car axles)

[Guest]

I have 4:10 on 33" and am happier than when I had 3:07 on 33" I think for off road, 4:56 is the best choice for 33". So, with 32" tires, you would be happy with 4:10 gears. Some Jeeps came with 4:10 and maybe you could find some axles at the junk yard and just bolt them up. I have '89 axles with my '93 Wrangler

Your local, so you can swing by for a test spin and see if you like the 4:10 on 33" tires.

[headhunter]

An attempt at making sense of all of the pieces to the puzzle...

Here is a chart that you may find helpful:

http://www.4lo.com/calc/geartable.htm

It gives you a ball park for tire sizes and their corresponding gear ratios.

There are a couple of other formulas on that page you might find helpful as well.

Here's how I personally think through this issue of choosing the right gears. This may work for me, but someone else may have a different set of parameters in mind. I've mentally beat this to death and have come up with what I think is logical for my application. I'll go through my thought process in a detailed manner from beginning to end, so that it's easy to track.

Since I drive my Jeep to where I'm going to be wheeling and don't trailer it, I want to have a good ratio in 4th gear (1:1) for driving at 65 mph. This will be important to me if I'm driving the Jeep from the LA area to Silverton, CO, Moab, UT or NorCal's trails in the Sierra's. Some of those drives will be 12 - 18 hours on the highway. It becomes less important if I'm only driving for about an hour and a half to the trailhead.

I'm currently riding on tiny 29" tires in my stock Scrambler. I'm turning about 3,000 rpm @ 65 mph. I've got 4.10 gears, because this Jeep originally came with a 4 cyl. engine and they tried to make up for the lack of power with lower gears. With the 6 cyl. I could use a slightly higher gear in my differentials on the highway, but I really like the ratio on the low end with these small tires.

For reference, 1st gear on my SR4 transmission is 4.07:1. Lot's of transmissions have a 1:1 4th gear, but they have very different ratios in 1st gear. This will become more important to this train of thought in a minute.

When I upgrade to 35" tires, I'm thinking of going with either 4.10 or 4.56 gears in the differentials on wider stronger axels to keep my highway rpm about 200 rpm lower than I have now. That will put me at about 2,800 rpm @ 65 mph. I believe that this will be optimal for the highway.

Now that we have the highway end figured out, we can think about how to get what we want on the off road end.

The off road measure for the cabability of your your gearing is usually expressed as your crawl ratio. I argue that you don't have to give up the highway ratio to accomodate a low crawl ratio. This way of thinking will maximize your mileage when driving to the trail in 4th gear, but will then let you have a crawl ratio that will satisfy your needs off road.

My Dana 300 transfer case has a 2.62 low gear. My 1st gear in my transmission is 4.07 and my current differential gears are 4.10. This gives me a crawl ratio of 44.

That's fine with my tiny 29" tires, but with 35" tires won't be as good off road. By changing my differential gears to 4.56 my crawl ratio will go to 49, but I want an even better crawl ratio.

I could change my differential gears to 4.88, which would give me a crawl ratio of 52. This slightly sacrifices my highway rpms and still doesn't give me the crawl ratio I'd like to see.

Instead, I can swap my weak SR4 transmission for a bulletproof SM420. This still gives me a 1:1 4th gear, but lowers my 1st gear to 7.05:1. With 4.56 gears in my differentials it will preserve what I want on the highway and changes my crawl ratio to 84, which is now very respectable.

If I were to change the gearing in my transfer case, I could still keep my high range at 1:1, which would not affect my highway rpms. Here is how the crawl ratio would change when using some of the standard low gear ratios that come on an Atlas transfer case assuming that I am running 4.56 gears in the differentials and the SM420 transmission.

3.0:1 = crawl ratio of 96
3.8:1 = crawl ratio of 122
4.3:1 = crawl ratio of 138

I can vary the end result by changing different things in the equation. For instance, if I go with 3.8 gearing in my transfer case, but use 4.10 gears in my differentials, I'll get a crawl ratio of 110.

If I go with 4.3 gearing in my transfer case with 4.10 gears in my differentials, I'll get a crawl ratio of 124.

4.10 gears are physically larger than 4.56 gears, so I believe that they would actually be stronger. This would lead me to believe that using 4.3 gearing in the transfer case with 4.10 gears in the differentials would be a stronger combination than using 3.8 and 4.56 gears respectively. This also gives me a slightly better crawl ratio.

My understanding is that crawl ratios in the 90s should be extremely capable, above the 110 mark should be very impressive and above 130 should be beyond the diminishing rate of marginal returns.

At this point, I need to make one thing perfectly clear. My entire train of thought does not come from practical experience, but from theory and mathematical calculations. I've spoken with lots of people and read lots of information and this is what I've come up with, so far.

I invite those with actual practical experience in this arena to correct anything that I've written here and point out any of the flaws I may have encountered in my logic. Practical experience will generally trump whatever I can conceive with a calculator.

My goal with this post is to attempt to help others and verify that I'm on track with my thinking at the same time.

So...what do all of you who have been out doing the real testing think about all of this? How close or far from hitting the mark am I?

Christian

[Guest]

I will double check the next time I drive the jeep and post what rpm I get at 65mph. With 4:10 and 33" tires. Well see if the math is right.

[Double AA]

Christian,

You are going through the same thought process everyone goes through before re-gearing. You spend hours looking through gear charts plugging in different scenarios.

Keep in mind all of these changes are factors you will feel once modified;
1. Spinning larger gears and shafts in your trans, transfer case and axles.
2. Added vehicle height (aerodynamics)
3. Larger tires (added rolling resistance)
4. Added weight.

As you know, re-gearing only moves the speed where you make the power and the power loss is still there.

Having said that, without overdrive, 48's and 35's are real low in top gear on the freeway. 4:58 would be more livable. Remember, the 4.2 doesn't like higher RPM's.

You didn't say which trails you're aiming this project at. If you are planning on spending most of your time in areas like Big Bear 4;58's and your stock D300 are fine.

If you're planning on spending a lot of time in the rocks, you'll never go too low. The disadvantage of a low transfer case is when you don't need it, you'll hit top speed quicker and need to shift into high range. My D300 was a SOB to shift when stock and after it was a 4 to 1. The Atlas is much easier to shift from low to high and back. If an aftermarket TC is in the budget and with the long CJ8 wheelbase, I'd go with the 4 speed Atlas.

Another disadvantage is if you're coming off an obstacle and get tippy, you may need to point the tires down hill and hit the throttle. If you're in too low of a gear, you may not have the wheel speed to drive out of the situation. I personally wouldn't worry about this when selecting a low range transfer case.

If it were me, and driving on the highway for extended periods of time, without overdrive, was a consideration, I would run 4:58's and a 4 to 1 kit in the D300. With a set of 35's and a good suspension system, you'll be set.

[2blk2drs]

What about if you throw a 6 spd MT into the equation? Can the overdrive gears not handle the torque?

[aw12345]

6 speed is nice trans, fairly nice 1 st gear ratio and not a lot of overdrive. Just does not shift as nice as some of the older 5 speeds

[OU812]

Isn't the six speed the same ratios as the five speed with one more overdrive gear?

[headhunter]

I will double check the next time I drive the jeep and post what rpm I get at 65mph. With 4:10 and 33" tires. Well see if the math is right.

Do you have an overdrive or is your top gear 1:1?

Christian

[aw12345]

Ron you have a 5 speed so it has an overdrive in 5th
The 6 speed has less overdrive than all the 5 speeds
Just smaller steps between the gears in other words one more gear between first and fifth.
Christian an SM420 or a NP435 or a SM465 would all make a nice trans for you. What Double A wrote is the way to go, it all depends on what you want to do with your Scrambler occasional rock play stick with the stock transfer case frequent rock play go 4 to1 or of you have the coin go Atlas or stack 3 or 4 speed transfer case.
That way you can pick a low range that makes for comfortable driving for the situation at hand

[headhunter]

Andrew,

You bring up some very good points to consider in addition to the "math."

I'm actually thinking of going with the Atlas 4 speed, because it will give me more usable low ranges for different activities. The low ranges I'm considering with that are 2.0:1, 2.72:1, and 5.44:1. They also offer 2.72:1, 3.8:1 and 10.3:1, but that seems silly if I'm not going to get larger than 35" tires.

With 4.10s and the SM420 trans, here is what the crawl ratios come out to with the various low ranges offered in the Atlas 4-speed:

2.0 = 58
2.72 = 79
3.8 = 110
5.44 = 157
10.3 = 298

Here are the crawl ratios with 4.56s:

2.0 = 64
2.72 = 87
3.8 = 122
5.44 = 175
10.3 = 331

I do want to play in the rocks, but will probably spend more time out in the wilderness on an "expedition style" trek. This doesn't mean that I'll be sticking to trails that are no more difficult than a graded fire road. In the case of trails like the Rubicon, Dusy Ershim and Fordyce, I'll still need the rock crawling ability provided with really low gear ratios.

It seems that if I were to decide on the Atlas 4-speed that 4.10s may be a better way to go. It keeps my highway rpms at 65 mph at about 2565, which I think would be kinder to the engine on long highway drives to a trail. On the trail, I'd still be able to acheive a crawl ratio of 157, which seems to me (from a purely academic standpoint) incredible for playing in the rocks.

By the way, I'm completely open to being wrong about this.

All of this underscores the importance of having some discussions with people who have had real world experience with what I'm simply going over in theory.

These decisions will require laying down a substancial amount of coin and it would be a bummer to not get it right, because of some important issues that were not considered due to lack of experience.

That's really the main reason I've posted what I have in this thread. I want the original poster and others who may stumble across this thread to realize that there is a lot to consider when looking at the big picture and final build.

Any input from those who have already gone down this road is really valuable to those of us that are just getting started.

Thanks.

Christian

[headhunter]

Ron you have a 5 speed so it has an overdrive in 5th
The 6 speed has less overdrive than all the 5 speeds
Just smaller steps between the gears in other words one more gear between first and fifth.
Christian an SM420 or a NP435 or a SM465 would all make a nice trans for you. What Double A wrote is the way to go, it all depends on what you want to do with your Scrambler occasional rock play stick with the stock transfer case frequent rock play go 4 to1 or of you have the coin go Atlas or stack 3 or 4 speed transfer case.
That way you can pick a low range that makes for comfortable driving for the situation at hand

Art,

I get the feeling that I'll want to spend a lot more time rock crawling when my Scrambler becomes more capable and more importantly when my skill level increases substantially from doing a lot more wheeling at all the various stages of my build with all of you. I have a lot to learn that I can't learn on the Internet. It will only come by getting out there and trying it and by listening to the seasoned veterans who have been there and done that.

The real bottom line is that time spent in the rocks will most likely be proportional to the size of the "war chest" I build up in my "recreational" bank account for repairs. Right now that dictates that I stay out of the rocks, but I have a plan to fix that!

Thanks.

Christian

[aw12345]

Well Christian you can do this in steps, maybe go up in tires some with a set of 32's or 33's stick a 4-1 kit on the Dana 300. Then progress from there, it will make it a lot more suitable for some somewhat more serious fire road recreation