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Hello everyone


Jimmayy13

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Jan 5, 2022
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8
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Nashua, NH
I'm new here. I'm gonna be buying a jku an was wondering how bad is the 3.8l because I found a nice white 2010 sport. Plan on building it up specially 37" tires. Any thoughts
 
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Vman

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Aug 23, 2021
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192
Location
Pacific NW
I'm new here. I'm gonna be buying a jku an was wondering how bad is the 3.8l because I found a nice white 2010 sport. Plan on building it up specially 37" tires. Any thoughts

If the front differential is a dana 30, especially if it is an automatic, you won't be able to run 37's unless you dump money at it.

If it's an automatic with a dana 30, you can run 35's but you'll need to re-gear to 5.13's. Ask me how I know. ;)
 
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Vman

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Aug 23, 2021
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Pacific NW
@Vman what do you have for a jeep? Is the 3.8l auto good for off an on road with 33's. Curious if it will rev high on highway

I have an 07 3.8 auto...so same as you basically. Mine has a dana 30 front diff which I think all the "x's" have, and is a 4 door unlimited.

There's a few things working against you with an 07-11: 4 speed automatic transmission, the 3.21 or 3.73 gearing in the diffs, a little less horsepower from the 3.8 compared to the 3.6, and for the non-Rubicons the dana 30 diff in the front.

Once you start throwing on larger (heavier) tires, the gearing in the diff and the shift points for the 4 speed get really bad. Even with 35's my JK would struggle going up a grade/hill and the transmission was shifting all over the place. I'd have to turn off overdrive to stop it from going bonkers going up a hill. Putting 37's on a dana 30 is a lot of weight that could break your axle just hitting a pothole.

Here is a great link to compare the size of tire you want to run, with either auto or manual, and if you're running a 3.8 (07-11) or a 3.6 (12+) JK. Blue would be a trail only rig, green is fine for highway driving, and yellow is underpowered and bad for both. It lists what your RPM's would be at 70 mph I think.


I can say that my '07 unlimited x (sport) on 35's with 5.13 gearing runs great. Transmission shifts naturally, you don't get the loss of power feeling, and the RPM's at all ranges are not a problem.
 
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Vman

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Aug 23, 2021
Messages
192
Location
Pacific NW
Thanks. On highways how is your jeep with 35's. I'm looking to go either 33 or 35 for now

It runs great. I have a 7 mile grade with a 2000 ft elevation change between home and work and have zero problems keeping it at 65-70. Got it re-geared about a year ago and the difference was night and day, even for fairly flat driving. Cost was about $1600 for parts and labor to get the gearing for both axles done. Money well spent imho. That included a few extra things like getting a rear axle seal replaced though.

Do you live near a 4x4 shop? You may want to discuss with them as they could determine what gears you currently have and maybe give you some advice.
 

Vman

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Aug 23, 2021
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192
Location
Pacific NW
Ya there's a couple here in NH

The other thing I did when they did my gearing was get eaton truetracs installed. Didn't notice you're in NH but I can say the truetracs work great in snow ;)

We've been getting hammered with snow lately here and I can say I'm very happy with how they perform in the snow.

Since I was getting my gearing done I had them replace the carriers with the truetracs. Think both of them were about $1k for front and rear. Didn't cost me anything in labor since they were already doing my gears.

Good luck with your build! This is stuff most of us have been through lol.
 

Vman

JK Enthusiast
Joined
Aug 23, 2021
Messages
192
Location
Pacific NW
@vmando you offroad your rig? If so no complaints of it?

Yeah I do some weekend warrior stuff, fishing and camping trails that other people are going to have problems getting into. There's a lot of interesting places to get lost in WA, ID, OR and MT. I also enjoy just daily driving it and crawling over parking lot barriers for some mall crawling.

It's a good blend of get you where you want to go but still be able to daily it. The 5.13's and the truetracs have been working great in Snowmageddon too, which has turned even the main roads into offroad driving lately.
 
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