Need help shopping for new gears

AndyG

TJ Addict
Supporting Member
Jul 30, 2018
1,542
Alabama
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.
This leads to my suspicion that it is actually the CV joint causing these vibes in some cases ....particularly the ones that are resolved by removing the front shaft .
 
Reactions: Jerry Bransford
OP
LCL-Dead

LCL-Dead

It broke.. ..again
Supporting Member
Mar 21, 2017
151
Havelock, NC
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.
This is not how an automatic transmission works.
 
Reactions: Jerry Bransford

Alex01

Enthusiast
Supporting Member
Ride of the Month Winner
Mar 23, 2018
1,462
Martinez, CA, USA
This leads to my suspicion that it is actually the CV joint causing these vibes in some cases ....particularly the ones that are resolved by removing the front shaft .
@bobthetj03 has had 2 different front drive shafts (new). I think its safe to say at least in his case he can rule out front cv joints.
 

AndyG

TJ Addict
Supporting Member
Jul 30, 2018
1,542
Alabama
@bobthetj03 has had 2 different front drive shafts (new). I think its safe to say at least in his case he can rule out front cv joints.
Yes, I agree .

Man that has to be frustrating. I screwed up my gears the first attempt, but at least I knew the general problem. These vibe issues seem so elusive.
 
Reactions: Alex01

TJ Hunnicutt

TJ Enthusiast
Supporting Member
Apr 5, 2018
136
Texas, USA
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.
Woah, Chris, name calling...what...where????
 
Reactions: Alex01

Alex01

Enthusiast
Supporting Member
Ride of the Month Winner
Mar 23, 2018
1,462
Martinez, CA, USA
Yes, I agree .

Man that has to be frustrating. I screwed up my gears the first attempt, but at least I knew the general problem. These vibe issues seem so elusive.
Fortunately for most it doesn't seem like a common issue.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
Hurts the engine? Haha... wow... some people are so stupid.

And in addition to that, if you want good fuel economy you shouldn't even own a Jeep / truck / 4x4.

Honestly, it amazes me how dumb some people can be.
In this specific example Chris is talking about high RPM operation. Chrysler puts all of its engines at the lowest possible RPM under light load. Everybody has a right to their opinion including Chris, but when the people that design your own Jeep feel extended high RPM operation isn't great you aren't exactly dumb to agree with them. On this thread you might be in a minority but certainly not dumb or stupid. I've got a thick skin, it doesn't bother me but it is just a good example of berating the person and not the idea.

There is no guesswork or estimates in gearing, there is no logical conclusion you have to make. There is only a calculated gear ratio at a given speed, it's mechanical advantage. If you don't want to consider that, then don't. Really, if an automatic transmission is a big black mystery box to you then don't worry about it.

Just like Mr. Bills posting the links about specific troubles people have. I don't think that he is saying don't re-gear, just that you should go into it knowing what you are getting into.

Gearing discussions are interesting, especially when people have something to add to the discussion.
 

pagrey

TJ Enthusiast
Apr 10, 2018
170
Los Angeles, CA, USA
Automatic transmissions up-shift to their top gear, they never "hold" a lower gear or allow the engine to gently spin up to a higher RPM. Under light loads automatic transmissions all reduce RPM as much as they possibly can. The 42RLE even disconnects the torque converter downhill off throttle letting the engine go to idle instead of letting it spin up any higher than necessary.
 

Alex01

Enthusiast
Supporting Member
Ride of the Month Winner
Mar 23, 2018
1,462
Martinez, CA, USA
According to grimmjeeper at 75 mph with my 4.88 gearing I am at 2558 rpm. With my 4.10 gearing it would have been 2149 rpm.

I didn't notice any improvement or decline in MPG after my regear. I reset my trip at every fill and calculate the mpg and I consistently get 13mpg. What I did notice though was the benefit of the gearing while off-road and especially in 4low.
 

JMT

The Jeep Guy
Supporting Member
Ride of the Month Winner
Automatic transmissions up-shift to their top gear, they never "hold" a lower gear or allow the engine to gently spin up to a higher RPM. Under light loads automatic transmissions all reduce RPM as much as they possibly can. The 42RLE even disconnects the torque converter downhill off throttle letting the engine go to idle instead of letting it spin up any higher than necessary.
The 42rle does do that (as do other transmissions), but why did the Chrysler engineers design the tranny to do that? Is it really because higher rpm's are not good for the engine? Is an engine-tranny combination right just because it runs lower rpm's? Why is the 42rle almost unanimously hated for it's 0.69 overdrive?
 

Chris

Administrator
Staff Member
Sep 28, 2015
34,475
Salem, Oregon
The 42rle does do that (as do other transmissions), but why did the Chrysler engineers design the tranny to do that? Is it really because higher rpm's are not good for the engine? Is an engine-tranny combination right just because it runs lower rpm's? Why is the 42rle almost unanimously hated for it's 0.69 overdrive?
My understanding (and perhaps someone more knowledgable like @mrblaine can comment on this as to whether I'm wrong or not) was that Chrysler used the 42RLE for the TJ because it was a transmission they were already using in many of their other FWD vehicles. It was never a clean sheet design or anything, specifically meant for the TJ.

In addition, I've read things here and there in the past that mentioned the ridiculous overdrive ratio and constant low RPMs it keeps you at were a result of Chrysler trying to meet emission standards.

Again, I don't know if that's accurate, it's just what I've read.

I really don't care who says what, a 4.0s optimal cruising RPM on the highway is certainly not under 2000 RPM. I mean sure... if you want to live with that and think that's acceptable, then by all means, be my guest. I think the majority will agree that it's mind numbing.

So yes, while everyone is entitled to their "opinion" on the matter, I think that the majority of people are going to agree that keeping the engine at higher RPMs (above 2000) is going to be better.

In addition, I've heard from @Jerry Bransford and others that consistently keeping the engine at low RPMs thinking that you're "saving the engine" is actually bad for the engine. I'm not the expert on this matter, but it makes sense from things I've read over the years.
 

jodomcfrodo

TJ Addict
Supporting Member
Feb 24, 2016
1,150
Evanston, IL, United States
Running for a long time at "high" RPM does not kill engines. What leads to engine failure is overloading. Assuming the running conditions are acceptable (IE clean oil), you could run an engine with no load at redline for damn near forever. Once you start putting too much load on the engine, then you can cause failure. This is the reason lugging the engine is a terrible idea. Lugging is essentially putting too much of a load on an engine, basically expecting it to do too much. I think some people assume that engines have a hard limit as to how many times they can rotate before they give out. That is not true at all.
 
Reactions: Chris

Chris

Administrator
Staff Member
Sep 28, 2015
34,475
Salem, Oregon
Which is even more reason I'd avoid an installer that wanted me to install a gear ratio that kept me at or under 2000 RPM at highway cruising speed. At those RPMs your lugging the engine, which isn't going to do any good for it.
 

AndyG

TJ Addict
Supporting Member
Jul 30, 2018
1,542
Alabama
I wonder why every time there is a thread on octane, lift geometry or regearing before it's over someone is dragging someone's fat alcoholic mother into the conversation.

I'm sorry but I think most of the time the person that is quoting all the statistics or math or whatever has the least experience of actually doing it... And I think that's just to support the fact that they really don't know what they're talkin about..

anyway I'm not trying to hurt anybody's feelings and I may be as wrong as I can be so accept my apologies if I am... And I don't think I'm the Forum police...but all you have to do is take the Jeep down to the shop and tell them to put the right ratio of gears for the tire size in it and write a check.

It's more important to deal with a competent shop than it is to know all the statistical probability.

Anyway that's my rant... I'll shut up.
 
Reactions: Cole and JMT

JMT

The Jeep Guy
Supporting Member
Ride of the Month Winner
Just so we're clear. My little 2.5 4 banger, even though it sounds like it may explode, is actually happier around 3 - 3200.

PERFECT.

For the rest of this, I'm just here for the comments. :)
Exactly. When I had my 3.73's and 6-speed with 33's I often used to run in 5th around 3000rpm just so I could keep speed on the hills around here. 6th was non-existent. The engine loved it and my temps never went over 210. Now that I'm on 4.56's I'm at 2850rpm in 6th at 70mph and the engine loves it, I have power, my top speed is lower, but I don't know what it is because I'm not interested in running my Jeep over 75mph. If I'm going to go that fast I'll just drive my father-in-laws C6.
 
Reactions: KCsTJ