Need help shopping for new gears

psrivats

TJ Addict
Supporting Member
Jun 4, 2018
1,232
OR, USA
That's the first time anyone has mentioned a front-end vibration strictly from the choice of 5.13 gearing in any of the several Jeep forums I follow. Any vibration is more likely from an improper front pinion angle than the actual specific 5.13 gear ratio itself. When that's the case it can usually be dialed out by correcting the pinion angle. I wouldn't hesitate going with 5.13 at all for 33" tires, I went to 5.38 for my 35" tires.
Jerry, with due respect, if you haven't experienced these harmonic vibes, you really don't know how difficult a problem it is. It's not a simple pinion angle or driveshaft balance problem. I've spent the last 5 months dealing with it and trying a LOT of things (just ask Dave Kishpaugh). Scores of forum members including @bobthetj03 @Mr. Bills @Chris and myself have been trying to figure it out.

Deeper than factory gearing somehow brings the problem to the forefront in some jeeps. People have reported issues with 4.56/4.88/5.13/5.38s. In some jeeps, locking front hubs is the only solution. In others, like mine and @bobthetj03's, even that doesn't work.

Please don't trivialize this problem. Your advice for gear choices is spot on (maybe on the deeper side, if any), but please note that regearing TJs has an inherent risk in terms of introducing harmonic vibrations. For some of us, there is no free lunch.
 
Last edited:

Chris

Administrator
Staff Member
Sep 28, 2015
34,475
Salem, Oregon
Also note that there is a big difference between a vibration (i.e. bad pinion angle) and a harmonic / cyclical vibration.

I've experienced both, and the latter is almost unreal at speed. If it was possible to view it on an oscilloscope, it would be very, very pronounced.
 

Rob5589

Certified video trained differential rebuilder
Supporting Member
Aug 29, 2016
6,022
W Sacramento, Kalifornia
According to grimmjeeper 5.13 gears will shift into third at about 65 leaving the Jeep with a worse ratio than the stock Jeep which is in second until 81, so between 65 and 81mph 5.13s are worse for passing power. Not a huge deal but it is there to consider.
That makes absolutely no sense.
 

jodomcfrodo

TJ Addict
Supporting Member
Feb 24, 2016
1,150
Evanston, IL, United States
Jerry, with due respect, if you haven't experienced these harmonic vibes, you really don't know how difficult a problem it is. It's not a simple pinion angle or driveshaft balance problem. I've spent the last 5 months dealing with it and trying a LOT of things (just ask Dave Kishpaugh). Scores of forum members including @bobthetj03 @Mr. Bills @Chris and myself have been trying to figure it out.

Deeper than factory gearing somehow brings the problem to the forefront in some jeeps. People have reported issues with 4.56/4.88/5.13/5.38s. In some jeeps, locking front hubs is the only solution. In others, like mine and @bobthetj03's, even that doesn't work.

Please don't trivialize this problem. Your advice for gear choices is spot on (maybe on the deeper side, if any), but please note that regearing TJs has an inherent risk in terms of introducing harmonic vibrations. For some of us, there is no free lunch.
Remember the scope of things. While you may have vibes, the vast majority of people who regear just don't find these issues. And if they do find vibrations, they are usually fixed by something small like the pinion angle. Just because a couple people are fighting the vibration battle doesn't mean that there needs to be a word of warning everytime someone wants to regear. Thats how you blow a small problem out of proportion.
 

Mr. Bills

TJ Enthusiast
Nov 24, 2017
978
Area Code 530
Remember the scope of things. While you may have vibes, the vast majority of people who regear just don't find these issues. And if they do find vibrations, they are usually fixed by something small like the pinion angle. Just because a couple people are fighting the vibration battle doesn't mean that there needs to be a word of warning everytime someone wants to regear. Thats how you blow a small problem out of proportion.
I don't think that anyone on this forum has taken the position that "every," "most," a "vast majority" or even a "large number" of TJ's re-geared to 4.88 or deeper suffer from vibrations or harmonics that went undetected before. Minimizing the issue by characterizing it as only affecting "a couple of people" is also misleading. The accurate state of affairs is "some do, some don't."

For those who do have the issue it is very real. As has been mentioned, sometimes pinion adjustment helps, sometimes messing with the crossmember/skidplate to change its natural harmonic helps, sometimes it just isn't worth the time and energy to try to tune the vibration/harmonic down or out and installation of manual hubs is the solution that quickly resolves the issue and makes driving one's jeep fun again.

Posting what has worked and what hasn't worked provides a resource for those who re-gear in the future and who may experience the issue. In other words, it allows people to hope for the best but prepare for the worst. Pretending that the issue doesn't exist or that it has never been mentioned isn't productive. Neither is being a Chicken Little. As with many subjects discussed on Internet jeep forums, stepping back from the discussion enough to view the various points of view in their proper perspective is key.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
That makes absolutely no sense.
Just gear ratios, the math isn't that hard? Getting on the Freeway at exactly 65 mph the Jeep with the 5.13 gears will be about redline and shift into third so now it is at lower RPM and worse total gear ratio. The Jeep next to it with stock gears is not only at a higher RPM making more power at 65mph it is also in second gear with a TOTAL gear ratio lower until it reaches 81mph when it finally shifts into third. Is that more clear-er? Just because you put 5.13 gears in doesn't mean you magically have more power everywhere, you have to consider the transmission ratios and the speed at which you are driving.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
This is a huge simplification but maybe this shows what I'm getting at. The shaded regions, which are not even remotely to scale, are speeds at which a higher numerical final drive like 5.13s might cause you to be slower or have less power available at the wheel. For me this is completely a non issue so if you are serious about these gears do the math yourself. Grimmjeeper!

76592
 

jodomcfrodo

TJ Addict
Supporting Member
Feb 24, 2016
1,150
Evanston, IL, United States
Just gear ratios, the math isn't that hard? Getting on the Freeway at exactly 65 mph the Jeep with the 5.13 gears will be about redline and shift into third so now it is at lower RPM and worse total gear ratio. The Jeep next to it with stock gears is not only at a higher RPM making more power at 65mph it is also in second gear with a TOTAL gear ratio lower until it reaches 81mph when it finally shifts into third. Is that more clear-er? Just because you put 5.13 gears in doesn't mean you magically have more power everywhere, you have to consider the transmission ratios and the speed at which you are driving.
I understand completely what you're saying, but there is simply no reason to consider this when you regear. Do you typically drive around with it floored? Do you think that a Jeep with 3.73's is going to beat out a Jeep with 5.13's in a race because of this? You also have to remember that the 4.0 makes peak power at 3000 rpm, not at redline. Try to put that into your model and think about it. You'll end up with torque ratios at different RPM compared to the gear ratios that put you at those RPM. It becomes too complicated to think about very quickly. If you think that through, you might find that the 5.13 equipped Jeep has more passing power from 65-81.

There is a lot of variables that you have to take into account with this idea which makes it hard to put it all together to get a concrete answer. The final answer is that putting the correct gear ratio in is better for drive-ability and for average power.
 

Rob5589

Certified video trained differential rebuilder
Supporting Member
Aug 29, 2016
6,022
W Sacramento, Kalifornia
Just gear ratios, the math isn't that hard? Getting on the Freeway at exactly 65 mph the Jeep with the 5.13 gears will be about redline and shift into third so now it is at lower RPM and worse total gear ratio. The Jeep next to it with stock gears is not only at a higher RPM making more power at 65mph it is also in second gear with a TOTAL gear ratio lower until it reaches 81mph when it finally shifts into third. Is that more clear-er? Just because you put 5.13 gears in doesn't mean you magically have more power everywhere, you have to consider the transmission ratios and the speed at which you are driving.
You do not choose gearing based on shift points, generally. You gear based on final drive/top speed/optimum rpm for the engine and trans combo. I understand you are using math to come up with your assumptions, so your condescension isn't necessary.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
You also have to remember that the 4.0 makes peak power at 3000 rpm, not at redline. Try to put that into your model and think about it. You'll end up with torque ratios at different RPM compared to the gear ratios that put you at those RPM. It becomes too complicated to think about very quickly. If you think that through, you might find that the 5.13 equipped Jeep has more passing power from 65-81.
Peak horsepower on my 4.0 is @ 4600rpm and adding that to the math doesn't help things for the 5.13 gears. These are facts that people should consider and can check for themselves, not my opinion. I used a flat graph so it would be easier to understand the basic idea.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
You do not choose gearing based on shift points, generally. You gear based on final drive/top speed/optimum rpm for the engine and trans combo. I understand you are using math to come up with your assumptions, so your condescension isn't necessary.
I have listed several concrete points that people often overlook when selecting gears for the 42rle, what's your point? I'm taking about real issues and you are taking about useless generalizations like top speed. For those that understand the math this will be useful, for those that don't it will not be. I don't really care which category anybody is in.
 

jodomcfrodo

TJ Addict
Supporting Member
Feb 24, 2016
1,150
Evanston, IL, United States
I have listed several concrete points that people often overlook when selecting gears for the 42rle, what's your point? I'm taking about real issues and you are taking about useless generalizations like top speed. For those that understand the math this will be useful, for those that don't it will not be. I don't really care which category anybody is in.
No, you're talking about useless tidbits that have no relevancy in the real world. There is a reason no one talks about this, because it doesn't matter (not because they don't understand it). What is your overall point? That we should put stupidly tall gears in because we might have more passing power in second at the same speed we would be in third with deeper gears? 3.73 x 1.54 / 5.13 x 1.00 yields 1.12. So you have a 12% better gear ratio in your scenario. What is that worth? What about if we start talking about a 5 speed or 6 speed? Then your scenario falls apart. You have zoomed in on one very specific issue of regearing a 3 speed. At the end of the day, cruising RPM and torque when taking off from a start are much more important than passing power from 65-81 in terms of drive-ability.

You don't have a valid point. But we are all very, very, impressed with your math skills.
 
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Rob5589

Certified video trained differential rebuilder
Supporting Member
Aug 29, 2016
6,022
W Sacramento, Kalifornia
I have listed several concrete points that people often overlook when selecting gears for the 42rle, what's your point? I'm taking about real issues and you are taking about useless generalizations like top speed. For those that understand the math this will be useful, for those that don't it will not be. I don't really care which category anybody is in.
Nobody is talking outright top speed. You gear for the top cruising speed, eg, 75 on the fwy. Not how fast can my Jeep go.
So, using your formulas, what gearing would you recommend for the 42 and 33's or 35's?
 

Pacoson71

New Member
Jan 27, 2019
4
MA
My numbers with 285/ 75 R16s (32.8") 42 rle and 5.38 ratio puts me on point with a factory spec
4.10 ratio NSG 370 6 speed.
5.38 is what I'm ordering for My 33s
 
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Mr. Bills

TJ Enthusiast
Nov 24, 2017
978
Area Code 530
As is often said, opinions are like a**holes, everyone has one.

My opinion for the 4.0 engine and 42RLE transmission, based on my personal observations and driving experience with 4.88 gears mated with 35" tires, 5.13 gears with 35" tires, and 5.38 gears with 35" tires, is that 5.38 gears are the best choice for 35" tires. However, my opinion is also that 5.13 gears would probably work as well or better than 5.38's with 33" tires everywhere except mountain roads with frequent elevation changes and steep grades . However, if there is any chance of going from 33" to 35" tires in the future, my opinion is that 5.38 gears would be the better long term purchase.

YMMV
 

AndyG

TJ Addict
Supporting Member
Jul 30, 2018
1,542
Alabama
Let me tell you what gearing you need in your Jeep TJ...

What actually works.


Professionals like Rob do not need the analysis it takes to land the space shuttle in order to get to where they want to be with a gearing job... That is all academic.

When you do it for a living , practical experience and general reference points will tell you what you're looking for.

Not to mention years of customer feedback.

I'm a contractor . if you tell me based on calculations that a 100 square foot floor laid on diagonal requires 104.5 ft, I'm probably going to tell you to get a hundred and ten feet or whatever even cartons comes closest to that, and it will work everytime .

When I regeared my TJ , I simply asked what most people did with my same set up, went with what the majority said ....and I am thrilled.
 

JMT

The Jeep Guy
Supporting Member
Ride of the Month Winner
When I regeared my TJ , I simply asked what most people did with my same set up, went with what the majority said ....and I am thrilled.
x2. well I did the math myself and put the numbers in at GrimmJeeper, but I got the same answer. 4.56, 33's NSG-370, 4.0. Every day driving is way better than the 3.73's
 
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Jerry Bransford

TJ Guru
Supporting Member
Nov 9, 2015
9,461
Escondido California
First, might get vibes, thousand of dollars to fix vibes. Might ask Jerry why he just replaced his driveshaft, I'm sure it was unrelated.
Second, 5.13 will force you to deal with that horrible overdrive everyday. Getting on the freeway your Jeep will be slower than it is now when it shifts out of third gear because your current Jeep can stay in third to much higher speeds. This means your passing power at 60 will be reduced, just simple math.
Third, you can just hit the o/d button and have almost the same final drive without thfat horrible o/d gap.
Do what makes you happy but don't let anybody sweep the details under the rug. There are big upsides on the trail but there are other things to consider. Read threads, when people like Chris just resort to name calling there might be more to the story.
There are a lot of very knowledgeable people here who completely disagree with how you're arriving at your conclusions. Me included. No one here understands your logic, me least of all. The ratios we recommend when running the 42RLE are based on lots of real-world experience on what available ratios work best. You're the first I've ever seen say that things would be worse with 5.13. Worse than what ratio? The factory 3.73? 4.88 that makes the engine lug? I'm not following your logic or reasoning at all and I was never the slowest guy in class.

And I didn' t replace my driveshaft for any reason other than it was cheaper to have Tom Wood provide a new one than to have it rebuilt. It was old enough that I started getting nervous about its CV joint. Having the front CV seize can literally destroy the tcase and it's far better to rebuild the front CV or replace the entire driveshaft before that happens. I was having zero drivetrain vibrations with my 5.38 gears before I replaced the driveshaft and I'm still having drivetrain zero vibrations with my 5.38 gears.