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Old 11-10-2015, 01:03 PM
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Regear with lift

I have a 2015 JKU sport with 3.21 and an automatic transmission. I am going to put in an AEV 2.5" lift and looking to put on 35" tires. My question is will I need to re gear and to what. I will be using it mostly as a dd and very rarely will go off road. What are your recommendations?

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Old 11-10-2015, 01:23 PM   #2
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The automatic transmission covers up a lot of the inadequacy of 3.21 gears so a lot of people don't really notice how bad it really is. If you're not paying attention, you don't notice that the transmission never gets into overdrive and it never locks up the torque converter.

The optimum gears for 35's are either 4.56 (low altitude, no big mountains, light towing) or 4.88 (high altitude, mountains, towing). Those gears put your engine in the optimum RPM range on the freeway so that you take maximum advantage of overdrive and the lockup torque converter to get optimum fuel mileage.

This is why: Regearing Basics

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Old 11-11-2015, 11:25 PM
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The automatic transmission covers up a lot of the inadequacy of 3.21 gears so a lot of people don't really notice how bad it really is. If you're not paying attention, you don't notice that the transmission never gets into overdrive and it never locks up the torque converter.

The optimum gears for 35's are either 4.56 (low altitude, no big mountains, light towing) or 4.88 (high altitude, mountains, towing). Those gears put your engine in the optimum RPM range on the freeway so that you take maximum advantage of overdrive and the lockup torque converter to get optimum fuel mileage.

This is why: Regearing Basics
Thanks Derf! I think I'm going to lift it and see how the ride is. If it is really a big difference then I will re gear. Thanks for the info!
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Old 11-12-2015, 05:48 AM   #4
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Thanks Derf! I think I'm going to lift it and see how the ride is. If it is really a big difference then I will re gear. Thanks for the info!
Trust me-its a huge difference. You will be running less than 1800rpm-ish at Highway speed, overdrive will be useless. I am properly geared for my tires - I run 2800rpm-ish at Highway speed. Huge difference.
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Old 11-12-2015, 09:55 AM   #5
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Thanks Derf! I think I'm going to lift it and see how the ride is. If it is really a big difference then I will re gear. Thanks for the info!
Get up to speed on the freeway and pay attention to your tachometer. With the automatic running 35's and 3.21 gears, if you find that the RPMs are around 2,300-2,500 at highway speed you're still in 4th gear and not in overdrive. And that means the torque converter is not locking up and you're leaving 1-2 MPG on the table as a result.

4.56 gears with 35's puts you in the 2,600-2,800 range around 70-75 but you'll be in overdrive with the torque converter engaged and the engine will be running right where it's making really good power (i.e. running most efficiently). You'll find you actually get slightly better mileage with this setup than with 3.21s. And it really improves off-the-line performance so you don't have to gun it so hard to get it moving from a stop.
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Old 11-13-2015, 10:55 PM   #6
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The optimum gears for 35's are either 4.56 (low altitude, no big mountains, light towing) or 4.88 (high altitude, mountains, towing). Those gears put your engine in the optimum RPM range on the freeway so that you take maximum advantage of overdrive and the lockup torque converter to get optimum fuel mileage. This is why: Regearing Basics
Thanks for laying this out so clearly. Problem I'm having is figuring out whether it's worth it for me to regear from 4.10s ('13 auto, 35s) and go to 4.56 or 4.88. I'm in Houston which is flat as a pancake and I don't tow anything, but seems like a lot of money just to go from 4.10 to 4.56
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Old 11-13-2015, 11:16 PM   #7
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If you had the 373 gears I would say do it, 410 on flat terrain should be fine. Both my rubicons were loaded with 373 and it was just too sluggish on the hwy in od here in hilly pa. I would run on the hwy in 4th with my auto. My 15 I just installed 456 which are perfect with my bfg km2's my 13 had toyo open country mt's and I ran 488 in that jeep. Run the 410's and spend the money elsewhere.
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Old 11-13-2015, 11:42 PM   #8
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Thanks for laying this out so clearly. Problem I'm having is figuring out whether it's worth it for me to regear from 4.10s ('13 auto, 35s) and go to 4.56 or 4.88. I'm in Houston which is flat as a pancake and I don't tow anything, but seems like a lot of money just to go from 4.10 to 4.56
4.10's are acceptable for 35's, especially if you have an automatic. Though with a manual, you still wouldn't be unhappy in and around Houston with 4.10's and 35's. You wouldn't notice much difference going to 4.56's so that would be a waste of money. And unless you're towing a lot or you put a bunch of really heavy accessories on a JKU (like skid plates, winch bumper with big winch, metal fender flares, etc.), 4.88's might be pushing more than you need down in Houston.

The one argument for going to 4.88's is if you do a lot of rock crawling. But from what I remember about Texas wheeling from when I lived there, you'd have to drive quite a ways to find any significant rock crawling. Though it's only, what, four and a half hours to Katemcy from Houston? And Barnwell (with less real rock crawling) is 4 hours or so. Though Disney, OK would be a full day's drive for you one way.
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Old 11-14-2015, 07:17 PM   #9
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We did 513s on my last jeep and they are right. If you're going to mostly drive this on the highway then stay to the 456 gearing. You will also be switching out the carrier when you regear as well. You could get your planned mods done now and drive it to see if you want the regear, or get it all done at once. I found out mine was too high after the install and that was something I was just not doing again.
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Old 01-08-2016, 12:53 AM   #10
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4.10's are acceptable for 35's, especially if you have an automatic.
x2

My torque converter finally shit the bed this week running 35s on stock 4.10s since Oct 2011. Making the big jump to 5.13s to have room for 37s down the road, course the wife might leave me if I raise the Jeep any higher.

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