Need Help Figuring Out Which Gears To Go With On 38s.

[JlURT]

Hello Jeepsters,

So, I really want to re-gear my JLURT 8-Speed Auto 2.0T but I can't figure out which gear would be better for 38s with an additional 250lbs of steel , 4.88s or 5.13? I would like to stay as short or maybe slightly shorter than factory as I don't really need to go over 70MPH in FL.

To help me decide and narrow down my question, I would like to know what rpms will I sit at with both 4.88s and 5.13s, at speeds of: 65MPH, 70MPH and 75MPH as 75MPH will probably be the fastest constant speed I'd go.

Any other Pointers or insight will be much appreciated.

Vendors please contact me as I'm looking to buy my set of gears sooner than later.

Here a picture of my rig for those that may be interested. I'm thinking of making a build thread, any request?

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[IronScott]

TrailRecon posted a good video on this recently. Good for me at least. He uses a formula to figure out the proper gearing. It’s around 4:00 on the video.

(New size / old size) * current gearing
38 / 33 = 1.15 * 4.10 = 4.72

Of course the actual size of the stock KO2 is a bit smaller so it would make sense to use the actual size of both tires but this seems like a logical formula.

 

[JlURT]

TrailRecon posted a good video on this recently. Good for me at least. He uses a formula to figure out the proper gearing. It’s around 4:00 on the video.

(New size / old size) * current gearing
38 / 33 = 1.15 * 4.10 = 4.72

Of course the actual size of the stock KO2 is a bit smaller so it would make sense to use the actual size of both tires but this seems like a logical formula.

Doing the math points me to 4.88s and that will probably be the gear I go with but I would like to get some feedback and also see how the rpms plays out at those speeds listed above. That will ultimately be the deciding factor. Also with the added weight was another thing I wanted to factor in.

 

[jruss]

TrailRecon posted a good video on this recently. Good for me at least. He uses a formula to figure out the proper gearing. It’s around 4:00 on the video.

(New size / old size) * current gearing
38 / 33 = 1.15 * 4.10 = 4.72

Of course the actual size of the stock KO2 is a bit smaller so it would make sense to use the actual size of both tires but this seems like a logical formula.

Grimm jeeper has a great fear ratio calculator. You have to do some homework beforehand and learn a bit more about gear ratios, transmission gearing, transfer case ratios, daily driving grades and cruising rpms to make a good decision.
In reference to this video, I can understand where this guy is coming from but he is missing some key points. If you are flatlanding most of the time and aren’t running a super heavy tire and wheel combination then 4.56 are probably the best all around gear for a later auto or early manual jk. 4.88 would suit a later manual jk. An 07-11 jk auto gets 5.38’s. Why? The transmissions are geared differently and have a massive impact on the behavior of the Jeep before the gears are calculated.
There is no be all end all solution on gearing so like I said, you will need to do some homework or prepare for disappointment.
My recommendation was for 5.13’s but that’s based off of the assumption you have an auto. I haven’t really looked too deep into the manual’s gearing.

 

[IronScott]

Doing the math points me to 4.88s and that will probably be the gear I go with but I would like to get some feedback and also see how the rpms plays out at those speeds listed above. That will ultimately be the deciding factor. Also with the added weight was another thing I wanted to factor in.

Yeah, that seems to be very important. Bigger might be better but if you drive a lot of daily miles it might not make financial sense or even for wear and tear with higher revs (I have no idea about that last part).

I have a Sahara with 33s (KM3) but may go to 35 next and will look at new gears, so I’m trying to figure this out myself. I had no idea what I was getting into when I decided to buy a Jeep. LOL.

 

[IronScott]

Grimm jeeper has a great fear ratio calculator. You have to do some homework beforehand and learn a bit more about gear ratios, transmission gearing, transfer case ratios, daily driving grades and cruising rpms to make a good decision.
In reference to this video, I can understand where this guy is coming from but he is missing some key points. If you are flatlanding most of the time and aren’t running a super heavy tire and wheel combination then 4.56 are probably the best all around gear for a later auto or early manual jk. 4.88 would suit a later manual jk. An 07-11 jk auto gets 5.38’s. Why? The transmissions are geared differently and have a massive impact on the behavior of the Jeep before the gears are calculated.
There is no be all end all solution on gearing so like I said, you will need to do some homework or prepare for disappointment.
My recommendation was for 5.13’s but that’s based off of the assumption you have an auto. I haven’t really looked too deep into the manual’s gearing.

That’s good to know. Thanks. I know very little about this but thought the video was useful. I may have to do this myself in the future so the more I can learn the better off I’ll be.

 

[JlURT]

Grimm jeeper has a great fear ratio calculator. You have to do some homework beforehand and learn a bit more about gear ratios, transmission gearing, transfer case ratios, daily driving grades and cruising rpms to make a good decision.
In reference to this video, I can understand where this guy is coming from but he is missing some key points. If you are flatlanding most of the time and aren’t running a super heavy tire and wheel combination then 4.56 are probably the best all around gear for a later auto or early manual jk. 4.88 would suit a later manual jk. An 07-11 jk auto gets 5.38’s. Why? The transmissions are geared differently and have a massive impact on the behavior of the Jeep before the gears are calculated.
There is no be all end all solution on gearing so like I said, you will need to do some homework or prepare for disappointment.
My recommendation was for 5.13’s but that’s based off of the assumption you have an auto. I haven’t really looked too deep into the manual’s gearing.

I did go to Grimm jeepers website and looked at everything needed to calculate and needless to say it looks like alot. But the more you know the more you grow. I was hoping a experience person or perhaps a vendor might shime in.

Edit: not to say your information wasn't expertise. It definitely was which is why I'm still thinkin of the 5.13 as an option. Based on the votes so far I will have to look into this more thoroughly.

 

[jruss]

I did go to Grimm jeepers website and looked at everything needed to calculate and needless to say it looks like alot. But the more you know the more you grow. I was hoping a experience person or perhaps a vendor might shime in.

Edit: not to say your information wasn't expertise. It definitely was which is why I'm still thinkin of the 5.13 as an option. Based on the votes so far I will have to look into this more thoroughly.

I know a ton of information about jeeps and 4wd vehicles. I made my living on them for several years. I do not know a ton of information about your Jeep though, and every Jeep is unique to its owner once the modifications start. Hopefully digging in a little deeper and learning more will help you become an expert on your Jeep and keep you from learning things the expensive way like I had to. That is why I try to be somewhat general in my replies. Hopefully I helped. If not then disregard.

 

[Tech Tim]

To help me decide and narrow down my question, I would like to know what rpms will I sit at with both 4.88s and 5.13s, at speeds of: 65MPH, 70MPH and 75MPH as 75MPH will probably be the fastest constant speed I'd go.

In 8th gear (.67 ratio)
With 4.88 gears:
65 = 1879
70 = 2024
75 = 2168

With 5.13 gears:
65 = 1975
70 = 2127
75 = 2279

In 7th gear (.84 ratio)
With 4.88 gears:
65 = 2356
70 = 2537
75 = 2718

With 5.13 gears:
65 = 2477
70 = 2667
75 = 2858

Drive around using the manual shift points at those RPMs and see what you like.

It's only about 100 rpm difference between the 4.88 and 5.13 at cruising speeds.

 

[b1er]

I did 4.88's with 37's and it's perfect for me... either will be good...

 

[ECHO]

I had a 2dr JK 3.8 v6 standard transmission with 5.13 gears and rarely used 1st gear and highway driving killed my gas mileage.. If I had a do over I would go 4.56 or 4.88..

My last Jkur had 4.10’s as well as my current JLR .. I like it better on and off road

 

[chevymitchell]

If you come to the conclusion that the RPM difference between 4.88 and 5.13 isn't a concern, then go with the 4.88. It'll be better to have the thicker pinion as insurance for when you're wheeling. I put 4.88's in mine and it was the best mod to date.

 

[TJEli]

What is your reason for wanting to re-gear? I have been driving and wheeling the crap out of mine on 4.10s with 38s and have not been disappointed. I am also less likely to cause axle damage.

-Eli

 

[Tech Tim]

What is your reason for wanting to re-gear? I have been driving and wheeling the crap out of mine on 4.10s with 38s and have not been disappointed. I am also less likely to cause axle damage.

-Eli

@TJEli, the JLs do run pretty dang good with the 4.10s because of the gearing in the 8spd auto, however you'll be impressed if you ever do swap out to a lower gear. It'll bring the engine back to its proper operation RPM to gear range and drives so much better.

You are probably more likely to do axle damage. The lower (numerically) gamer ratio will make it easier to turn the tires from a dead stop, that equates to less strain on parts.

 

[OffroadTreks]

Ever put a 4.88 and 5.13 pinion side by side? The 5.13's will give you a touch more grunt, but with the weaker pinion, you could likely damage the teeth. I've seen it happen to several friends.

I'll probably go 4.88's personally. That being said, like you, I'm running the 2.0T and I've been very impressed with it's ability to spin my 37's compared to how the older 3.6 in the JK felt which was like a turd.

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