Why such deep gearing recommendations?

[greaseorbounce]

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?

 

[89grand]

If you have an overdrive, you should be able to use it, and people regear to that overdrive generally. My Jeep has the 32rh spd, with no overdrive, and the ideal gear for mine to make power at highway speeds is 4.10. 4.10 is lower in final drive in my Jeep, then 4.88's would be in yours in final drive. 4.88 x .79 = 3.85 final gear ratio, 4.56 x .79 = 3.60. 5.13 with the overdrive would be 4.05 final drive.

The 2003 and up auto's have a really bad .69 overdrive ratio.

Gearing is expensive, you never want to pay for it, and realize you didn't go low enough.

 

[pagrey]

The gearing suggestions listed in the Resources section on this forum aren't just random selections. They are based on what most people that have these things find best, lots of miles and lots of different owners. Of course you can choose whatever gearing you want but most of the really knowledgeable people on this forum are just going to echo what is already clearly stated everywhere else. Take it or leave it basically. You can punch in numbers at http://www.grimmjeeper.com/gears.html if you want something different. You wont be the first person to pick a different ratio than what most people suggest but give it some serious thought before you ignore the advice of so many people.

 

[greaseorbounce]

The gearing suggestions listed in the Resources section on this forum aren't just random selections. They are based on what most people that have these things find best, lots of miles and lots of different owners. Of course you can choose whatever gearing you want but most of the really knowledgeable people on this forum are just going to echo what is already clearly stated everywhere else. Take it or leave it basically. You can punch in numbers at http://www.grimmjeeper.com/gears.html if you want something different. You wont be the first person to pick a different ratio than what most people suggest but give it some serious thought before you ignore the advice of so many people.

Yeah, I guess maybe it wasn't clear in my post, but I am trying to better understand the situations and experience that have led these people to these suggestions. I am not saying I disagree, I am also not saying that I plan to ignore them. I also meant no disrespect to these suggestions. I merely am trying to learn WHY and better understand the experiences these people have had. I'm aware they are not random selections, they are founded in experience and math. But I am trying to understand what that experience actually looks like. I'm trying to educate myself, not argue with people.

 

[billiebob]

it's on 33s, and it is currently geared 4.10s

I'm in complete agreement, I have a TJR, 33s 4.10s, I wish for 3.73s but then I drive mine 24K miles a year. The right gear for you depends on how, where you drive.

Most of the guys are making recommendations for TJs seeing mostly rugged trail use, or limited pavement use. And the gearing recommendations are like they were 50 years ago for a street/strip muscle car. One goal of acceleration, all other goals be damned. 4.10 was the holy grail for a drag car.

Best rpm for me in the mountains is 2200rpm at 60mph. And I drop a gear, or 2, or 3 for the mountain passes and towing a trailer. Gas mileage goes to hell if your cruise is over 2500rpm.

As you get bigger tires, ie 35s, 37s, you will want higher cruising rpms since all that tire robs power like crazy. With 37s I have no doubt I'd want a 3000rpm highway cruise. Altho, I'm not sure why I'd be on the highway with 37s.

Same thing with the SE 4 cylinder. There is a lot of reserve in the 4.0L with 33s. Zero reserve in the 2.5L with 32s. The bigger the tire, smaller the motor, the closer you need to be to peak power cruising. The less room there is for compromize.

Semis on the Interstate run pretty much at the governed rpm. Pedal to the metal just to hold 70mph. Fully loaded and running wide open.

 

[Chris]

Well with the 42RLE for instance, you want as deep as you can get in most cases, as the overdrive on that transmission is a real pain in the ass. Another good example is the 4 cylinder models, which are so anemic that usually gearing anything but the deepest gear you can go is just a waste of time and money.

I'd also take it from someone (me) who has not geared deep enough before, that when in doubt, you should ALWAYS go deeper with the gears. These engines can use all the gear they can get, especially with bigger tires. The optimal cruising RPM for the 4.0 at highway speeds is over 2000 RPM. Anything close to 2000 RPM or lower is simply not enough.

These aren't modern V6 (or V8 engines) with plenty of power on tap. The 4.0 puts down maybe 150 horsepower at the wheels if you're lucky. The deeper gears help to offset that to some degree.

 

[greaseorbounce]

Well with the 42RLE for instance, you want as deep as you can get in most cases, as the overdrive on that transmission is a real pain in the ass. Another good example is the 4 cylinder models, which are so anemic that usually gearing anything but the deepest gear you can go is just a waste of time and money.

I'd also take it from someone (me) who has not geared deep enough before, that when in doubt, you should ALWAYS go deeper with the gears. These engines can use all the gear they can get, especially with bigger tires. The optimal cruising RPM for the 4.0 at highway speeds is over 2000 RPM. Anything close to 2000 RPM or lower is simply not enough.

These aren't modern V6 (or V8 engines) with plenty of power on tap. The 4.0 puts down maybe 150 horsepower at the wheels if you're lucky. The deeper gears help to offset that to some degree.

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?

If I look at speed limits out here on big highways (the ones I have to run to get to trails, or the wonderful trip to moab) they are 75. In fact here in CO, law prohibits going less than 10 under the posted speed in some areas (or the posted minimum speed). So if I do the math on wanting 2500 rpm at 75 in 5th, I get 4.56 with 35s. Then that magic 2000rpm threshold happens at 58mph, so if you're traveling less than 58 you just don't use 5th gear.

I guess my biggest confusion in the matter is that I don't have any issues reserving 5th for interstates, and if I use the 2000rpm minimum as my point of reference, I wind up figuring lower gearing than the conventional recommendations.

I really appreciate your insight! This is exactly the discussion I'm looking for!

@billiebob your sentiments really echo my experience. I totally see where you're coming from with the limited power overhead comments. Makes perfect sense. Again, I also agree with you that the mid 2000s (2200, 2500, etc) seem to be the happy place cruising. Yet again, this is exactly the kind of input I'm looking for. Curious what other's have found as well!

 

[pagrey]

I merely am trying to learn WHY

I didn't take it negatively, personally I've never found an answer to that question.

 

[Steel City 06]

To give you an idea of what factory RPMs look like on the newer TJs, the 05-06 manual Rubicon came with 4.10 gears, ~31" tires, and a 0.84 OD. It turns over 2700 RPM on the highway at 70 mph and about 3100 rpm at 80.

And that's 100% stock. So assuming the Chrysler engineers knew what they were doing, deep gearing is not an issue for the 6 speed.

 

[billiebob]

if I do the math on wanting 2500 rpm at 75 in 5th, I get 4.56 with 35s

With 35s, you might want closer to 3000rpm at 75mph, but of course you can just drop a gear. With 35s you might do that often.

 

[jjvw]

Why is 3k rpm at 75rpm a screamfest?

 

[greaseorbounce]

With 35s, you might want closer to 3000rpm at 75mph, but of course you can just drop a gear. With 35s you might do that often.

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?


As to why I consider 3000 to be a screamfest, maybe I'm just old and cranky, or maybe this particular TJ needs new exhaust, but it seems to be that the difference between 2500 and 3000 is a significant jump in engine and exhaust noise, and takes it from comfortable conversation to yelling at your passenger and desiring ear plugs after a while.

 

[jjvw]

...


As to why I consider 3000 to be a screamfest, maybe I'm just old and cranky, or maybe this particular TJ needs new exhaust, but it seems to be that the difference between 2500 and 3000 is a significant jump in engine and exhaust noise, and takes it from comfortable conversation to yelling at your passenger and desiring ear plugs after a while.

That's not my experience at all.

 

[GASnBRASS]

If you have an overdrive, you should be able to use it, and people regear to that overdrive generally. My Jeep has the 32rh spd, with no overdrive, and the ideal gear for mine to make power at highway speeds is 4.10. 4.10 is lower in final drive in my Jeep, then 4.88's would be in yours in final drive. 4.88 x .79 = 3.85 final gear ratio, 4.56 x .79 = 3.60. 5.13 with the overdrive would be 4.05 final drive.

The 2003 and up auto's have a really bad .69 overdrive ratio.

Gearing is expensive, you never want to pay for it, and realize you didn't go low enough.

Same setup here (32rh, 33's) and I put in 4.10's on the advice of numerous people. While it's good for maintaining speed on a hill or accelerating around town, it gets a bit old cruising down a flat open hwy and having the motor needlessly spin faster than it really needs to. It begs for an overdrive, or at least one more gear. I've grown to dislike the huge drops in rpm between gears, and as much as I'd love to have a 5 speed I despise hydraulic clutches even more. Maybe someday I'll swap in an AW4.

 

[BuildBreakRepeat]

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?


As to why I consider 3000 to be a screamfest, maybe I'm just old and cranky, or maybe this particular TJ needs new exhaust, but it seems to be that the difference between 2500 and 3000 is a significant jump in engine and exhaust noise, and takes it from comfortable conversation to yelling at your passenger and desiring ear plugs after a while.

Sounds like you need a new muffler. 3000 rpm is near silent in my Jeep. And that’s the happy place of the 4.0

 

[eastbloc]

6 cyl 6 spd on 265/75r17 tires here.
I just went from 3.07 to 4.56, lord ohh mighty, what a difference. I feel like im learning how to drive a stick again.

Daily driving before the regear, i lost use of 6th gear. After the reagear, i lost use for 1st gear.
Engine is doing 65 mph at roughly 2750 rpm.

Could have i went with 4.10 and be happy? Yes of course, difference would have been negligible. However, 4.56 did open the doors for me to 33" if i choose that option one day, and with the way things are right now i feel like this ratio will be more than enough for it.

Does the engine screams at 3000rpm in 6th gear? - NO.
It is actually quieter than doing 2500rpm in 5th gear. I don't know what changes when 6th gear is engaged, but engine goes into a different mode, and difference in sound is there.
Engine actually feels much happier at 3000rpm than 2500rpm

Many believe that 6cyl 4.0 engine feels wrong above 2000rpm, so people want to keep the rpms down and will steer you into other direction with their suggestions. This is actually a reasonable (but misleading) belief, 4.0 does feel very stubborn in between 2000 and 2600 rpm gap, but below or above that gap it runs smooth.

Currently i am comfortably driving in 6th gear while doing 45mph. Little bit of gas, and it spiritedly goes faster. On the highway same thing: hills, heavy wind, no matter- little bit of gas and it goes from 60 to 80 with a smile, no need to downshift.
Would i chose 4.10 over 4.56 if i was to do it again? - NO
Would i chose 4.56 over 4.10 ? - NO.
Literally splitting hairs here for my current situation. But, as mentioned above, 4.56 did open doors for me.

All of us have different configurations, and usages for our Jeeps.
Some of us have no other option but to go deeper.
When one ask question, answers a question, or simply browsing the forum, must understand it.

You also have to understand that some don't know any better but to repeat what a "more popular/reputable" member have said before. I have browsed this forums for a long time before registering, and i have noticed this strong trend of "fanboy" among some members.
Its not that there is bad misinformation flowing this forums, its actually other way around, plenty of good info. Its just that some people repeat things that others have said without understanding why.

 

[Boinked]

Unfortunately there is no magic number that says X with Y is the end all be all of gears.......... no matter how many people say there is.

I can say i dont agree with the 4.88 gears with the ax-15. In "MY" situation. I live in fairly flat area but there are many mountains roads around.

At 75mph. My jeep does wonderful in 5th gear. If i need to climb a grade i drop to 4th. I don't think I would like 4.88 it would put my rpms up to high at 75.

However....... if you don't travel at 75mph and only drive 45-55. 55-65 ect... it would be worth the deeper gears.

I don't use 5th gear on city roads. I don't need to i hold 45-50 @2700-3000 (estimate) and it feels good.


Long story short...... the idea of one size fits all has been far to embraced. You need to look at your driving habits and conditions. Than consider what is going to be the best for "YOU".

 

[mrblaine]

I'm in complete agreement, I have a TJR, 33s 4.10s, I wish for 3.73s but then I drive mine 24K miles a year. The right gear for you depends on how, where you drive.

Bullshit.

Most of the guys are making recommendations for TJs seeing mostly rugged trail use, or limited pavement use. And the gearing recommendations are like they were 50 years ago for a street/strip muscle car. One goal of acceleration, all other goals be damned. 4.10 was the holy grail for a drag car.

Bullfuckingshit. No, the guys making the recommendations are making them so that the OD is tolerable on the god damn freeway, fucking off road be damned to hell and back.

Best rpm for me in the mountains is 2200rpm at 60mph. And I drop a gear, or 2, or 3 for the mountain passes and towing a trailer. Gas mileage goes to hell if your cruise is over 2500rpm.

Anyone reading your advice needs to pay close attention to the dropping of 2 or 3 gears.

As you get bigger tires, ie 35s, 37s, you will want higher cruising rpms since all that tire robs power like crazy. With 37s I have no doubt I'd want a 3000rpm highway cruise. Altho, I'm not sure why I'd be on the highway with 37s.

I have 32's, 4.88's, and the auto so no, not relevant to just the guys with bigger tires. The guys with 37's wish for even higher gears with the auto. And no, I don't doubt for a second you can't imagine 37's, not for a second.

Same thing with the SE 4 cylinder. There is a lot of reserve in the 4.0L with 33s. Zero reserve in the 2.5L with 32s. The bigger the tire, smaller the motor, the closer you need to be to peak power cruising. The less room there is for compromize.

Semis on the Interstate run pretty much at the governed rpm. Pedal to the metal just to hold 70mph. Fully loaded and running wide open.

We aren't semis, that is wholly irrelevant unless you swapped a 10-16 speed trans in there and didn't tell anyone.

 

[mrblaine]

You need to look at your driving habits and conditions. Than consider what is going to be the best for "YOU".

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.

 

[jjvw]

Unfortunately there is no magic number that says X with Y is the end all be all of gears.......... no matter how many people say there is.

I can say i dont agree with the 4.88 gears with the ax-15. In "MY" situation. I live in fairly flat area but there are many mountains roads around.

At 75mph. My jeep does wonderful in 5th gear. If i need to climb a grade i drop to 4th. I don't think I would like 4.88 it would put my rpms up to high at 75.

However....... if you don't travel at 75mph and only drive 45-55. 55-65 ect... it would be worth the deeper gears.

I don't use 5th gear on city roads. I don't need to i hold 45-50 @2700-3000 (estimate) and it feels good.


Long story short...... the idea of one size fits all has been far to embraced. You need to look at your driving habits and conditions. Than consider what is going to be the best for "YOU".

You are describing my driving environment. Mostly flat with mountains nearby. Most of my driving is on flat city streets followed by flat city highway.

Your comment about not driving 75mph, but 45-65 is very typical. The deep gears are great for that. Just like they are great for moving around at 65-75mph and sometimes 80mph.

 

[Brianj5600]

My TJ with 35s need more power for anything over 60 mph. It has just enough power at 70-75 mph, but none extra. I have 5.38s and it is a bit of a love hate thing. It does OK on the interstate at 70 mph and 2500 rpm, but when it is hot and humid with the A/C on the Tennessee hills will slow me down. Shifting out of OD raises the revs a little more than I like for longer climbs, and I'm talking empty. I would like to pull a small trailer, but don't see it happening without more power. With the stock 4.10s it had plenty of power in 3rd on the interstate at 70 mph around 2800 rpm. I'm likely going to build a stroker this winter and that should take care of things.