4.10's, 4.56's or stick with 4.88's?
03-11-2013, 03:23 AM
#31
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I am sure there have been at least several hundred threads regarding the same thing during the last 5+ years. I'd bet if you looked back, there is a regear thread at least every 3-5 pages in the Modified Tech Section.
The answers are always the same:
The 3.8L is designed to run at higher rpm levels than the old 4.0L. You could drive coast to coast at highway speed running 3500 rpm and it would not damage or overheat the 3.8L engine.
The automatic behind the 3.8L has a very tall 4th gear and needs to lowest possible gearing to approximate the same rpm in the 6 speed at highway speeds. (Which means 5.13s for non-Rubi's and 5.38s for Rubi's with 35s or taller tires if performance matters and you don't routinely "bounce" your rig with significant tire spin, locked in the rocks.)
Wind resistance from speed and drag from weight and tire tread design will affect your fuel economy more than an extra 100-200 engine rpm at highway speeds.
Larger, heavier than stock tires and heavier than stock armor and accessories need higher than stock engine rpm levels to produce more horsepower and torque to handle the extra weight and drag.
Many more people have paid the cost to regear twice to go to a lower gear (higher numbered gear) than those who have paid to regear the second time to a higher gear. My recollection is that there is one guy in Texas who went from 4.88s to 4.56s. However, there are dozens who have gone from 4.88s to 5.13s or 5.13s to 5.38s--particularly those with automatics.
In polls on this and other forums, regearing has received the highest tally of votes as the best modification for a JK with a 3.8L.
There is routinely a contingency in these threads who haven't regeared who share opinions about regearing with no real basis of comparison.
The FAQ even has a chart to show you mathematically your rpm levels at 70 mph, given your actual measures tire size and transmission type.
For some reason, there tends to be a contingency that doesn't trust the math and asks if real world rpm is different that the charts, or there are some who don't look at the charts and speculate that they must be running 5000 rpm or something with too low a gear.
For a 6 speed with 35s that measure an actual 33" or 34" when mounted on the rig with full weight on the ground, the difference between 4.56s and 4.88s at 70 mph is less than 200 engine rpm.
I do understand the concerns and why this topic gets re-asked over and over again. I've been there.
I made the mistake of regearing my 04 TJ years ago to 4.56s instead of 4.88s because I was afraid of high rpms--biggest mistake I made building that jeep.
The 08 4 dr JK automatic on 37s my wife drives had 5.13s that we later pulled to install 5.38s--we bought it already with the 5.13s installed.
But, this is simple math and physical science.
For a 6 speed on 35" all terrain tires, with very light armor/bumpers/mods driven at sea level on relatively flat ground, the 4.56s would be enough. They would be slightly preferable if the owner regularly drives long distances most every day at 75+ mph.
However, at 75 mph, fuel economy will suck due to wind drag, and an extra couple hundred rpm from running 4.88s will not result in meaningfully lower fuel economy.
At 80+ mph, the 4.56s would be more desirable because they would produce sufficient rpm to maintain speed (but only for lighter rigs that are flatlanders).
The 4.88s will be better in every way both on and offroad up unto that 70+ mph range. The 4.88s work the engine less because it doesn't have to work as hard. Just like when you ride a 10 speed bike with lower gears it is easier to pedal than lugging along in 10th gear.
The answers are always the same:
The 3.8L is designed to run at higher rpm levels than the old 4.0L. You could drive coast to coast at highway speed running 3500 rpm and it would not damage or overheat the 3.8L engine.
The automatic behind the 3.8L has a very tall 4th gear and needs to lowest possible gearing to approximate the same rpm in the 6 speed at highway speeds. (Which means 5.13s for non-Rubi's and 5.38s for Rubi's with 35s or taller tires if performance matters and you don't routinely "bounce" your rig with significant tire spin, locked in the rocks.)
Wind resistance from speed and drag from weight and tire tread design will affect your fuel economy more than an extra 100-200 engine rpm at highway speeds.
Larger, heavier than stock tires and heavier than stock armor and accessories need higher than stock engine rpm levels to produce more horsepower and torque to handle the extra weight and drag.
Many more people have paid the cost to regear twice to go to a lower gear (higher numbered gear) than those who have paid to regear the second time to a higher gear. My recollection is that there is one guy in Texas who went from 4.88s to 4.56s. However, there are dozens who have gone from 4.88s to 5.13s or 5.13s to 5.38s--particularly those with automatics.
In polls on this and other forums, regearing has received the highest tally of votes as the best modification for a JK with a 3.8L.
There is routinely a contingency in these threads who haven't regeared who share opinions about regearing with no real basis of comparison.
The FAQ even has a chart to show you mathematically your rpm levels at 70 mph, given your actual measures tire size and transmission type.
For some reason, there tends to be a contingency that doesn't trust the math and asks if real world rpm is different that the charts, or there are some who don't look at the charts and speculate that they must be running 5000 rpm or something with too low a gear.
For a 6 speed with 35s that measure an actual 33" or 34" when mounted on the rig with full weight on the ground, the difference between 4.56s and 4.88s at 70 mph is less than 200 engine rpm.
I do understand the concerns and why this topic gets re-asked over and over again. I've been there.
I made the mistake of regearing my 04 TJ years ago to 4.56s instead of 4.88s because I was afraid of high rpms--biggest mistake I made building that jeep.
The 08 4 dr JK automatic on 37s my wife drives had 5.13s that we later pulled to install 5.38s--we bought it already with the 5.13s installed.
But, this is simple math and physical science.
For a 6 speed on 35" all terrain tires, with very light armor/bumpers/mods driven at sea level on relatively flat ground, the 4.56s would be enough. They would be slightly preferable if the owner regularly drives long distances most every day at 75+ mph.
However, at 75 mph, fuel economy will suck due to wind drag, and an extra couple hundred rpm from running 4.88s will not result in meaningfully lower fuel economy.
At 80+ mph, the 4.56s would be more desirable because they would produce sufficient rpm to maintain speed (but only for lighter rigs that are flatlanders).
The 4.88s will be better in every way both on and offroad up unto that 70+ mph range. The 4.88s work the engine less because it doesn't have to work as hard. Just like when you ride a 10 speed bike with lower gears it is easier to pedal than lugging along in 10th gear.
03-11-2013, 04:24 AM
#32
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i know it would look awesome with a 3" lift and 35's, and I would be very happy with that, but what bothers me I guess is all this talk about a D30 breaking with gussets, sleeves and tuss's. Will I screw up with this build with 4.88's with keeping the D30? Like I said, I am not rough with my vehicles.
03-11-2013, 04:32 AM
#33
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There are several guys in our club running 37s on the stock D30 with 5.13s and no gussets or sleeves. These guys do some pretty tough crawling but know to stay out of the throttle when it gets rough. You will most likely break a tooth on the pinion or an axle shaft before the housing. I'd recommend you go with 35s, 2.5"+ lift, & 4.88s.
03-11-2013, 05:53 AM
#35
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There are several guys in our club running 37s on the stock D30 with 5.13s and no gussets or sleeves. These guys do some pretty tough crawling but know to stay out of the throttle when it gets rough. You will most likely break a tooth on the pinion or an axle shaft before the housing. I'd recommend you go with 35s, 2.5"+ lift, & 4.88s.
03-11-2013, 08:49 AM
#37
Exactly 
I know quite a few guys running 37s on stock D30s and D44s for 5+ years with no extra mods (not even gearing) and they've beat the shit out of their JKs without breaking an axle housing yet or gears. Some people break things and some don't.
4.88 and you need 35s or 37s if you have a 6spd and drive on the highway lots. Even 4.56 on the highway with 35s is nice as you drop a couple RPMs. I prefer to cruise the highway at 2200rpms and not listen to the Jeep scream, but offroad it requires lower gearing for rock crawling. We only do a couple strips a year around the states so I'm sure we can deal with the horrible gas mileage then, otherwise the Jeep is the fiancees daily driver and it's a long drive to work. 1hr+ right now at 50mph or so. The extra drive and gas will eat up her increase in wage for the new position but she loves her Jeep and wont drive the Honda to work to save money
If you don't mind the bad gas mileage go 4.88 and run the 33s till you decide on whether you want 35s or 37s hahaha. Don't worry too much about breaking down, it's a rarity and not very common. It's just this forum that makes people think they'll break everything and be stuck on the trail
I know quite a few guys running 37s on stock D30s and D44s for 5+ years with no extra mods (not even gearing) and they've beat the shit out of their JKs without breaking an axle housing yet or gears. Some people break things and some don't.4.88 and you need 35s or 37s if you have a 6spd and drive on the highway lots. Even 4.56 on the highway with 35s is nice as you drop a couple RPMs. I prefer to cruise the highway at 2200rpms and not listen to the Jeep scream, but offroad it requires lower gearing for rock crawling. We only do a couple strips a year around the states so I'm sure we can deal with the horrible gas mileage then, otherwise the Jeep is the fiancees daily driver and it's a long drive to work. 1hr+ right now at 50mph or so. The extra drive and gas will eat up her increase in wage for the new position but she loves her Jeep and wont drive the Honda to work to save money
If you don't mind the bad gas mileage go 4.88 and run the 33s till you decide on whether you want 35s or 37s hahaha. Don't worry too much about breaking down, it's a rarity and not very common. It's just this forum that makes people think they'll break everything and be stuck on the trail
03-11-2013, 10:07 AM
#38
Forum Tech Advisor
It is uncommon for JK axle housings to break.
It is common for the axle Cs and/or the passenger side tube to bend near the center section.
You can gusset the Cs for under $100.
For about $100, you can do an Artec truss and gusset the Cs.
The best I've seen are the TnT Customs full front axle trusses, but they do cost more.
Anyway, those guys who run 37s hard offroad won't notice a problem until they do an alignment check and their camber is way out of spec, or until they see front tire treadwear feathering.
For less than $100, I don't understand why anyone wouldn't at least gusset the upper axle Cs--particularly if they are in the process of regearing or upgrading ball joints anyway.
It is common for the axle Cs and/or the passenger side tube to bend near the center section.
You can gusset the Cs for under $100.
For about $100, you can do an Artec truss and gusset the Cs.
The best I've seen are the TnT Customs full front axle trusses, but they do cost more.
Anyway, those guys who run 37s hard offroad won't notice a problem until they do an alignment check and their camber is way out of spec, or until they see front tire treadwear feathering.
For less than $100, I don't understand why anyone wouldn't at least gusset the upper axle Cs--particularly if they are in the process of regearing or upgrading ball joints anyway.