Do I need to re-gear?

[Rango04TJX]

Hiya folks, I have a 2004 Jeep Wrangler X, 4.0 I6 5spd MT. It's my first jeep/truck/utility vehicle. I bought it used and it came with 31" Cooper A/T tires. I'm wondering if I need to or if I would benefit from regearing? The Jeep feels super sluggish and I can barely hold 55mph going up Donner pass. Is that just living with an old Jeep or should I regear? Would I see any difference in mpg? I am not planning on putting on larger tires for the forseeable future or lifting it. It's my daily that I take on pretty easy trails (signal peak and south lake tahoe mostly) for the occasional camping trip. The engine is bone stock other than a Magnaflow catback and a K&N air filter (not a cold air intake, just the filter). I've also had some people say that it's possible my cats are going bad... is there any way to test them without taking them off and driving without them? (edit I ordered an ACdelco air filter. I also have 3.07 gears)

 

[Steel City 06]

I would get rid of the K&N filter, first. They allow a lot of dirt into the engine and can shorten the life pretty dramatically.

Second, how are you shifting on these hills? If your engine is in good condition, keep downshifting as low as you need to. Peak horsepower on these engines is in the range of 4,600 RPM. So if you're not climbing fast enough with the pedal down, keep downshifting. Don't be afraid to hold it near redline for a minute or two to get to the top of the hill. I'll frequently hold 5,000 RPM going up hills on PA highways. (That said I will generally cruise a lot lower than that, generally 2,000 RPM street and 2,500-3,000 highway.)

Any idea what gear ratio you currently have? Stock is commonly 3.73

 

[Jerry Bransford]

Hiya folks, I have a 2004 Jeep Wrangler X, 4.0 I6 5spd MT. It's my first jeep/truck/utility vehicle. I bought it used and it came with 31" Cooper A/T tires. I'm wondering if I need to or if I would benefit from regearing? The Jeep feels super sluggish and I can barely hold 55mph going up Donner pass. Is that just living with an old Jeep or should I regear? Would I see any difference in mpg? I am not planning on putting on larger tires for the forseeable future or lifting it. It's my daily that I take on pretty easy trails (signal peak and south lake tahoe mostly) for the occasional camping trip. The engine is bone stock other than a Magnaflow catback and a K&N air filter (not a cold air intake, just the filter). I've also had some people say that it's possible my cats are going bad... is there any way to test them without taking them off and driving without them?

Ok #1 replace that shitty-filtering K&N with a good air filter like from AC-Delco, Purolator, etc. K&N air filters are absolutely the WORST at actually filtering out the harmful stuff like silica, dust, dirt, etc. You will not lose ANY power, none whatsoever, by replacing the K&N. The TJ's factory air intake with a conventional paper element air filter like from AC-Delco is not restrictive (!) will easily flow more air than the engine can consume at wide open throttle and redline rpms. The below is from an ISO 5011 certified lab test of various air filters. Notice the lab's last sentence below, bolded for emphasis.

But let's get to what is likely the reason for your lack of power, let's take a look at your axle ratio. If your TJ has the dreaded 3.07 ratio that's the reason. With the wrong axle ratio for the size tires being run your engine rpms will be too low and the engine won't be able to generate enough power for the size tires being run.

There's a tab bolted to your differential at its 9 o'clock position. The below tag indicates the gears inside the axle have a 3.73 ratio. What does your tag say? 3.07? 3.73? If it's a 3.07 you would pick up a significant boost in power by regearing to a lower axle ratio like 3.73 or 4.10. Regearing will raise the engine rpms up enough to produce enough power for your 31" tires.

 

[Duck Doctor]

My rigs that are properly geared for their tire size struggle going up Donner Pass. I can only hold 55 MPH in 4th gear.

 

[Jerry Bransford]

My rigs that are properly geared for their tire size struggle going up Donner Pass. I can only hold 55 MPH in 4th gear.

You're struggling up Donner Pass because you're only running 4.56 with 35's which is on the verge of lugging for that size tire. Really, 4.88 would have been a much better choice. I ran 35's and 4.88 with the AX-15 which is not geared quite as low as your 6-speed is and it did fine regularly going up to my local mountains. If you live at a higher altitude 5.13 would be a great choice too. Many would recommend 5.13 for your transmission and 35's even if you don't live at a higher altitude.

 

[Rango04TJX]

I would get rid of the K&N filter, first. They allow a lot of dirt into the engine and can shorten the life pretty dramatically.

Second, how are you shifting on these hills? If your engine is in good condition, keep downshifting as low as you need to. Peak horsepower on these engines is in the range of 4,600 RPM. So if you're not climbing fast enough with the pedal down, keep downshifting. Don't be afraid to hold it near redline for a minute or two to get to the top of the hill. I'll frequently hold 5,000 RPM going up hills on PA highways. (That said I will generally cruise a lot lower than that, generally 2,000 RPM street and 2,500-3,000 highway.)

Any idea what gear ratio you currently have? Stock is commonly 3.73

not sure i'll check the diff later today, I'm typically downshifting at 2200-2500 rpm so my rpm range is typically 2000-3600, engine is healthy I just don't like running it that high for a 45 minute drive, I've never had cooling problems though so I guess i just need to drive more aggressively

 

[Duck Doctor]

You're struggling up Donner Pass because you're only running 4.56 with 35's which is on the verge of lugging for that size tire. Really, 4.88 would have been a much better choice. I ran 35's and 4.88 with the AX-15 which is not geared quite as low as your 6-speed is and it did fine regularly going up to my local mountains. If you live at a higher altitude 5.13 would be a great choice too. Many would recommend 5.13 for your transmission and 35's even if you don't live at a higher altitude.

Jerry I have other rigs not just the one listed in my profile. My TJ with 5.38s and 35s also struggles up Donner pass. My XJ with 5.13s and 35s also has a tough time going over the Sierra Nevada mountains.

Agree the 4.56s were a mistake for my LJ. However, I haven’t tested that rig yet since it’s perpetually broken down.

 

[Rango04TJX]

Ok #1 replace that shitty-filtering K&N with a good air filter like from AC-Delco, Purolator, etc. K&N air filters are absolutely the WORST at actually filtering out the harmful stuff like silica, dust, dirt, etc. You will not lose ANY power, none whatsoever, by replacing the K&N. The TJ's factory air intake with a conventional paper element air filter like from AC-Delco is not restrictive (!) will easily flow more air than the engine can consume at wide open throttle and redline rpms. The below is from an ISO 5011 certified lab test of various air filters. Notice the lab's last sentence below, bolded for emphasis.

View attachment 452967

But let's get to what is likely the reason for your lack of power, let's take a look at your axle ratio. If your TJ has the dreaded 3.07 ratio that's the reason. With the wrong axle ratio for the size tires being run your engine rpms will be too low and the engine won't be able to generate enough power for the size tires being run.

There's a tab bolted to your differential at its 9 o'clock position. The below tag indicates the gears inside the axle have a 3.73 ratio. What does your tag say? 3.07? 3.73? If it's a 3.07 you would pick up a significant boost in power by regearing to a lower axle ratio like 3.73 or 4.10. Regearing will raise the engine rpms up enough to produce enough power for your 31" tires.

View attachment 452969

just looked, I do have the 3.07 gear ratio.

 

[Jerry Bransford]

just looked, I do have the 3.07 gear ratio.

That's what I thought, 3.07 is just a terrible ratio. Jeep made it a standard ratio thinking it would help the mpg rating but it has to be driven so much more aggressively that the 3.07 doesn't help the mpg in the least. I'd regear it to 4.10 which is the ratio the Rubicon came with with the manual transmission for its 31" tires. It'll be a great performing Jeep after regearing the axles to 4.10. Highway rpms will be ideal.

 

[Rango04TJX]

That's what I thought, 3.07 is just a terrible ratio. Jeep made it a standard ratio thinking it would help the mpg rating but it has to be driven so much more aggressively that the 3.07 doesn't help the mpg in the least. I'd regear it to 4.10 which is the ratio the Rubicon came with with the manual transmission for its 31" tires. It'll be a great performing Jeep after regearing the axles to 4.10. Highway rpms will be ideal.

sweet thank you I'll take a look into regearing. If i'm going to be cracking the diff open anyways is there anything else you guys would recommend? lockers or positrack or anything like that?

 

[machoheadgames]

I agree you need to regear, as 3.07 is terrible. However, I would chose 4.56 for 31’s and a 5-speed. I did 4.10 for a while and I always wished I chose 4.56.

 

[Jerry Bransford]

sweet thank you I'll take a look into regearing. If i'm going to be cracking the diff open anyways is there anything else you guys would recommend? lockers or positrack or anything like that?

Whether you need a locker or positrac (limited slip differential) depends on your needs. Do you get on trails that are uneven enough to lift a tire up off the ground? If so a locker is best. If your trails and needs involve flat trails or only slick/icy roads a limited slip differential is better.

Since your rear Dana 35 axle isn't strong enough for a locker, if you need one that is, I'd put a "lunchbox" locker like the Powertrax No-Slip into your front axle. That is if you actually need a locker. If you're pinching pennies you can go with a slightly less expensive lunchbox locker like an Aussie or a Powertrax Lockrite but they will make clicking/ratcheting noises during slow-speed turns when in 2wd like if you're maneuvering in a parking lot. Offroad it doesn't matter.

 

[Rango04TJX]

Whether you need a locker or positrac (limited slip differential) depends on your needs. Do you get on trails that are uneven enough to lift a tire up off the ground? If so a locker is best. If your trails and needs involve flat trails or only slick/icy roads a limited slip differential is better.

Do you know if your rear axle is the standard or the optional and stronger Dana 44? This is how both look...

This is a Dana 44. Notice its fill hole plug is a threaded steel bolt.

View attachment 452993

This is a Dana 35, notice its lubricant fill hole is plugged by a snap-in rubber plug.

View attachment 452994

I ask because a Dana 35 is strong enough for a limited slip differential but not a locker.

I have the Dana 35/30 combo, I have the X so it's pretty base apart from the I6. I definitely would benefit from lockers but i've heard a lot of mixed results from people locking the 35/30. some say it's fine as long as you don't bounce the axle but I've heard others say that they'll break if you look at them wrong.

 

[machoheadgames]

Dana 44’s didn’t come with 3.07 so that’s an easy one.

If you don’t do hard offroading, a TrueTrac limited slip rear can be advantageous over the stock open diff or clutch limited slip. If you do mild to medium offroading, a rear TrueTrac in combination with a front locker can be a good setup. For heavier wheeling front and rear lockers is best.

The Dana 35 should not be locked unless you go all the way with it and do a super 35 kit which upgrades to 30 spline shafts made of a slightly stronger material.

 

[Jerry Bransford]

I have the Dana 35/30 combo, I have the X so it's pretty base apart from the I6. I definitely would benefit from lockers but i've heard a lot of mixed results from people locking the 35/30. some say it's fine as long as you don't bounce the axle but I've heard others say that they'll break if you look at them wrong.

I just figured out you have a Dana 35 and edited my above post. You can lock your front axle as describe in my edited post above. It's strong for 33's and smaller tires. Or as suggested above, you can make your Dana 35 axle strong enough for a locker and even 35" tires with a Super 35 kit from www.revolutiongear.com which replaces the axle shafts in a Dana 35 with heat treated larger diameter axle shafts.

 

[deadbeat son]

I agree you need to regear, as 3.07 is terrible. However, I would chose 4.56 for 31’s and a 5-speed. I did 4.10 for a while and I always wished I chose 4.56.

And I regeared to 4.56 with 31s and a 5 speed and loved it. Great setup!

 

[GregBelleville]

I just figured out you have a Dana 35 and edited my above post. You can lock your front axle as describe in my edited post above. It's strong for 33's and smaller tires. Or as suggested above, you can make your Dana 35 axle strong enough for a locker and even 35" tires with a Super 35 kit from www.revolutiongear.com which replaces the axle shafts in a Dana 35 with heat treated larger diameter axle shafts.

Jerry, for a Dana 30, if I wanted to do a lunchbox locker, is it generally recommended to upgrade to chromoly axles too or could I get away with just swapping in the locker and leaving the stock axles as is?

 

[Jerry Bransford]

Jerry, for a Dana 30, if I wanted to do a lunchbox locker, is it generally recommended to upgrade to chromoly axles too or could I get away with just swapping in the locker and leaving the stock axles as is?

That depends on where and how hard you wheel. For 33's and most wheeling with a front lunchbox locker stock shafts are usually enough. For tougher wheeling like rock crawling or when 35's are installed 4340 Chromoly axle shafts are strongly recommended. I broke a factory front outer stub shaft rock crawling in Johnson Valley with 35's and lunchbox locker. No more problems there after replacing the front shafts with 4340 Chromoly.

 

[GregBelleville]

That depends on where and how hard you wheel. For 33's and most wheeling with a front lunchbox locker stock shafts are usually enough. For tougher wheeling like rock crawling or when 35's are installed 4340 Chromoly axle shafts are strongly recommended. I broke a factory front outer stub shaft rock crawling in Johnson Valley with 35's and lunchbox locker. No more problems there after replacing the front shafts with 4340 Chromoly.
View attachment 453083

I'm currently on 33s. With an open Dana 35 in the back and I'm contemplating the most cost effective way to increase my capability. I'd eventually like to do a super 35, but I'm curious what your opinion would be of doing a lunchbox in the front with an open rear? Is it generally preferable to have the locker on the rear if you only have one locker?

 

[Jerry Bransford]

I'm currently on 33s. With an open Dana 35 in the back and I'm contemplating the most cost effective way to increase my capability. I'd eventually like to do a super 35, but I'm curious what your opinion would be of doing a lunchbox in the front with an open rear? Is it generally preferable to have the locker on the rear if you only have one locker?

It's generally preferable to have the first locker in the rear but not when it's a stock Dana 35. In that case the first locker should go in the front axle where it will still provide significant benefits. A front locker by itself is still awesome to have while wheeling.