Why such deep gearing recommendations?

The best thing to find out is if they understand fully and one hundred percent that the FACTORY set the 4.0 up to run about 3000 rpm plus with no ill effects whatsoever and if doing so or getting close to that is going to somehow hurt your feelings because you think the motor doesn't like it.
I agree with this. 3k+ rpm isn't going to cause any problems. When i was doing my research on my gears i shot for the best torque @ the cruise speed i typically drive.

For the 97-99 4.0 it hits @2800 rpms.

I don't understand why but the 00-06 @ 3200.

Maybe you can explain why the torque curve is different. I can only assume that it has to do with the coil packs and removal of the distributor.
 
Maybe Blain or someone can let me know if my theory is wrong due to the torque converter in some way, but I calculated that in second gear on the freeway, it would be like having 4.72 gears. The second gear of 1.54 x 3.07. I found that while I can easily do 65-70mph in second gear, that does seem a bit too low, so I believe 4.10's will be nearly perfect, and why I'm having it regeared to that ratio.

I've had my Jeep to 4000 rpm a number of times when I had to shift to second to pass or maintain speed, and the engine showed no signs of hating it. Currently with 3.07's it sometimes cannot even maintain 65mph if there is a head wind at all, or the slightest incline, and mine runs around 2k at 65 in 3rd gear. It just makes no power at those speeds with that low of revs.
 
This makes sense to me! In the case of the 4cyl or the 42RLE, I totally get it. The big thing that still makes me scratch my head is that I consider interstate speeds here, and I look at charts, the recommendations for the 4.0 and AX15 seem odd. I completely agree about keeping the RPM over 2000. Anything below that and you're way out of the power and the engine is not happy. In my experience though, much over 3000 and she's just SCREAMING. So I tend to figure somewhere between 2000 and 3000 is best, and it seems that 2500 is about the most comfy. Is this similar to your experience?

With 35s on my TJ with the 42RLE and 5.38 gears, I was seeing around 2400 RPM I want to say at 65 MPH (which is average highway speeds for me). It's been a long time since I sold it to @toximus, so perhaps he can confirm if I'm far off from what I remember or not. But either way, assuming that's about where I was at RPM wise for 65 MPH, that's right where I wanted to be. I always figured that the fastest I ever felt comfortable going in my TJ was 85 MPH, and anything over that started to get seriously scary. 65 MPH for me was always what I liked to cruise at on the highway, and it never felt like the RPMs were screaming with 5.38 gears.
 
I'm tinkering with the idea of going lower than 5.38's. My 4.0, 5-sp. manual has 4.88's on 37's w/ a Dana 60/44 combo.
MT. has an 80 mph sp. limit. I'm more inclined to do 60-65 tops.
In my 4-Runner, 80 is a good cruising speed, or 90+ if you can get away with it.
 
I did a whole spreadsheet with a bunch of math, took data logs with my obd2 reader and app to generate a curve of wind and mechanical resistance vs speed and considered that I wanted the torque peak at the low end of highway cruise speed. I ended up with 4.56 with my Ax15 and 32s, knowing my next tires will be 33s. If I planned on going to 35s I would have gone 4.88. It would have been high for my taste but tolerable temporarily.

Let go of the fear of 3k rpm. I had a Mx5 with a high revving little 4 banger that spun 4k at 80 and it seemed less comfortable than the 4.0 does at 3k.
 
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My Jeep is not a DD, it is 100% for fun. I have 33's with a NV3550 and 4.56's. When I get in the deep dunes, the sand can suck power right out of it. I operate mostly in 1st and 2nd gear and there it is still torquey. I need both momentum and power for the bigger dunes. While 4.88's surely would be better suited, I'd probably be okay by dropping to a shorter tire.
 
I have a 97 with a 32RH auto 3 speed with 4.10's. 3000 RPM at 70mph, it feels great on the roads, trails, and highway, I wouldn't want anything different. When I had the same setup and stock gear I actually lost MPG because I had to floor it everywhere. I went from 9mpg in town to 14mpg in town just with a gear change. I am with the guys that say pick from the recommend chart if you want a well behaved ride.
 
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... I am with the guys that say pick from the recommend chart if you want a well behaved ride.
Just be wary that most printed charts I've seen always give the higher rpms you'd get in your 1:1 drive gear like 4th gear if you have a 5-speed. The Overdrive rpms you would normally cruise at would be 15-20% lower than what most charts indicate. Written ratio recommendation charts are, for the most part, the work of the devil... they are usually incomplete and/or misleading. Only for transmissions without an overdrive like your 3-speed 32RH automatic would most written charts be correct. :)
 
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Just be wary that most printed charts I've seen always give the higher rpms you'd get in your 1:1 drive gear like 4th gear if you have a 5-speed. The Overdrive rpms you would normally cruise at would be 15-20% lower than what most charts indicate. Written ratio recommendation charts are, for the most part, the work of the devil... they are usually incomplete and/or misleading. Only for transmissions without an overdrives like your 3-speed 32RH automatic would most written charts be correct. :)
Guess I got lucky :oops:
 
With 35s on my TJ with the 42RLE and 5.38 gears, I was seeing around 2400 RPM I want to say at 65 MPH (which is average highway speeds for me). It's been a long time since I sold it to @toximus, so perhaps he can confirm if I'm far off from what I remember or not. But either way, assuming that's about where I was at RPM wise for 65 MPH, that's right where I wanted to be. I always figured that the fastest I ever felt comfortable going in my TJ was 85 MPH, and anything over that started to get seriously scary. 65 MPH for me was always what I liked to cruise at on the highway, and it never felt like the RPMs were screaming with 5.38 gears.

I was looking at the tachometer the other day... it's been at 2100-2200 at 65 lately but I'm almost positive that it was previously at 2400 like you said. It could be the ECU learning and adjusting?
 
I was looking at the tachometer the other day... it's been at 2100-2200 at 65 lately but I'm almost positive that it was previously at 2400 like you said. It could be the ECU learning and adjusting?

Interesting! That could be, yes.

All I know is that even with the 5.38s, I never once felt like it was geared too deep. Hell, if it was possible to fit an even deeper gear in there, I probably would have done it. I have no hesitation at keeping the 4.0 at a higher RPM.
 
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Ah this changes things. If the consensus is that 3000rpm is a better place to be on 35s then I would better understand the advice. Admittedly I haven't played much on a TJ on 35s, so I may be off on thinking that 2500 would be nice. Has most experience pointed to 2500 being insufficient on that much rubber?

It all boils down to what RPM your engine performs best at. Here's some stock numbers
'96-'99 TJ: 181hp @ 4600rpm, 222lbft @ 2800rpm, redline 5300rpm
'00-'06 TJ: 190hp @ 4600rpm, 235lbft @ 3200rpm, redline 5300rpm
 
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I was looking at the tachometer the other day... it's been at 2100-2200 at 65 lately but I'm almost positive that it was previously at 2400 like you said. It could be the ECU learning and adjusting?
Your ecu can't change the physical gear ratio in your transmission or axles.
 
As to why I consider 3000 to be a screamfest, maybe I'm just old and cranky
I often drop a few gears. With a trailer I'll run 4000, 4500 on mountain passes. I'd never say the engine is screaming. I only like 2000-2200 rpm because the gas mileage is better. But there is so much other noise the engine is hardly a factor for me.
 
Alright, so "what gears should I use with xyz" gets asked daily, and everyone points to the same charts. I've looked at them, and I've used the grim jeeper calculator ad nauseam.

I really don't understand the driving factors behind some of the recommendations, and I'm trying to better understand where people are coming from.

It seems that everyone recommends gearing WAY deeper than I would have thought.

Here's an example: my current TJ has the AX15, it's on 33s, and it is currently geared 4.10s. Charts would suggest 4.88 ideal. HOLY CRAP! So I've considered maybe better with 4.56, but I can't imagine 4.88.

With the current setup, I have occasionally desired a lower crawl speed, but the 4:1 conversion seems like a better choice there. On the highway here at 75 I'm turning 2500rpm. That's pretty comfortable. Of course you have to downshift to pass, and sometimes on hills, but this doesn't seem that bad to me. Very rarely does it annoy. What am I missing?

Now I'm at the point that my next set of tires will be 35s. (Yes everything else is done to support them) I will definitely be regearing at that point, but with 35s all the charts would have me going with 5.13. This is nuts to me! That would have me turning 3000rpm on the interstate. Talk about a screamfest.

Where do these recommendations come from? Are people allergic to downshifting? Is 3000rpm on the interstate actually desireable? Or is it truly that people just REALLY want that lower crawl speed and sacrifice everything else? What am I not getting here?
I think what you may be missing is that with larger rolling mass you need deeper gears to turn that mass. Just re-gearing based on tire size alone (as if the rolling mass is a non-factor), will lead to undergearing. Your performance will not be the same as it was with the smaller tires. So you go deeper. I went with 456 for my 4.0/6-speed/33’s and I would not mind being at 488. I run 2850rpm on the highway and absolutely love it. I don’t have to downshift. I can zip by people at 80+ mph. I pulled a trailer 2,000 miles loaded up and still got 13mpg. No complaints.