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Last month I purchased a 2016 JKU Sport S that my dealer had modded with a 2.5" Rancho lift, 18" wheels and 34" tires. It's exactly what I had wanted, and will be living on Cape Cod and used strictly as a summer "about town" ride with occasional excursion over the "dunes of the Cape." Very little, if any, highway driving, no towing.

I understand that (existing gear X new tires)/existing tires = new gearing, would call for regearing to 3.73.

Given my use, what would be the impact if I don't regear from 3.21 to 3.73?

Thanks in advance for your responses.
 

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For 34 inch tires, "ideal" gearing would be 4.56, maybe 4.10.

You will have no or limited use of overdrive, added wear and tear on engine and transmission, you will lug at hills or steep inclines, accelerating will be a slug.

You are already undergeared on existing stock tires with 3.21, much less to go any bigger.

You paid a lot of money for your Jeep. Why not build it right and regear? I hate that dealers sell these modded Jeeps with gearing and big tires that can't get out of their own way - it is such a disservice to the customer.
 

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The most common answer I have seen to this is: you have to decide what you can live with and what you want to spend your money on. I have 34" tires and 3.21 gears, and it has been acceptable. But I have the lighter 2-door, and an automatic. The only thing I ever noticed that I don't like is that it shifts down on the highway in strong winds. If you'll only be driving around in lower gears anyway, you'll probably not notice until you want to go really slow.

One thing that may also be important is if you have an automatic, get a ProCal or similar device so the shift points are smarter.
 

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For 34 inch tires, "ideal" gearing would be 4.56, maybe 4.10.

You will have no or limited use of overdrive, added wear and tear on engine and transmission, you will lug at hills or steep inclines, accelerating will be a slug.

You are already undergeared on existing stock tires with 3.21, much less to go any bigger.

You paid a lot of money for your Jeep. Why not build it right and regear? I hate that dealers sell these modded Jeeps with gearing and big tires that can't get out of their own way - it is such a disservice to the customer.
. I agree with Mamma, skip the light bar, get the gears.
 

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For 34 inch tires, "ideal" gearing would be 4.56, maybe 4.10.

You will have no or limited use of overdrive, added wear and tear on engine and transmission, you will lug at hills or steep inclines, accelerating will be a slug.

You are already undergeared on existing stock tires with 3.21, much less to go any bigger.

You paid a lot of money for your Jeep. Why not build it right and regear? I hate that dealers sell these modded Jeeps with gearing and big tires that can't get out of their own way - it is such a disservice to the customer.
Is there a different formula you are using to come to the 4.10 or 4.56 gear ratio? I'm new to this and I'm just trying to figure this all out. I have read the same reports about how to get the right gearing that the OP described.
 

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4:10 would be optimal for 34" tires and acceptable for 35's particularly for what you are going to use it for. I have 34X10.5 KO2's and 3:21 gearing and it isn't that bad for what I do both on and off road...yeah it shifts on grades,and when fighting an oncoming wind,and lugs a bit in different situations,either just auto stick it into the next lowest gear or tap the accelerator to get 'er to shift down and problem solved. I know that 4:10 gearing would be absolutely perfect for my set up....and I'm seriously considering it. Although I wonder how 4:10's would be on the Dana 30 up front,no rock crawling,and only mud as I find it(not going out looking for it) just forestry service roads and the like. I guess the pinion gear(and ring) gets small as you gear down on the 30,and I just wonder how weak or fragile that would be off road.
 

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With 34" tires and 3.21 gears you'll definitely feel a reduction in overall performance over the Jeep in stock form.
 

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For 34 inch tires, "ideal" gearing would be 4.56, maybe 4.10.

You will have no or limited use of overdrive, added wear and tear on engine and transmission, you will lug at hills or steep inclines, accelerating will be a slug.

You are already undergeared on existing stock tires with 3.21, much less to go any bigger.

You paid a lot of money for your Jeep. Why not build it right and regear? I hate that dealers sell these modded Jeeps with gearing and big tires that can't get out of their own way - it is such a disservice to the customer.
I agree with this. I think the main reason dealers don't re-gear is because you can not "see" gearing. It would add to the cost something that someone that does not understand gearing/performance might be reluctant to pay for.
 

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4:10 would be optimal for 34" tires and acceptable for 35's particularly for what you are going to use it for. I have 34X10.5 KO2's and 3:21 gearing and it isn't that bad for what I do both on and off road...yeah it shifts on grades,and when fighting an oncoming wind,and lugs a bit in different situations,either just auto stick it into the next lowest gear or tap the accelerator to get 'er to shift down and problem solved. I know that 4:10 gearing would be absolutely perfect for my set up....and I'm seriously considering it. Although I wonder how 4:10's would be on the Dana 30 up front,no rock crawling,and only mud as I find it(not going out looking for it) just forestry service roads and the like. I guess the pinion gear(and ring) gets small as you gear down on the 30,and I just wonder how weak or fragile that would be off road.
Pinion size is usually not an issue till you go to 5.13's. Some won't go to 4.88, but 4.56's are ok, especially if you aren't hard on it. I beat on my d30 pretty hard with 4.56's. No issues so far....so far.


OP, if you regear, i'd do 4.56. I ran around with close to your combo for over a year. I wheeled with it too. It wasn't ridiculous, but not something I wanted to do long term. I will say it was night and day when I did regear.
 

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I was just talking to the place that would do my re-gear.

He told me that they do 4:56 gears for 33 up to 35 tires and 4:88 for 37's and lower gearing if you request,but for optimal gearing this is what they recommend-do. So I guess 4:56 for my KO2's @ 34X10.5 (33.2 actual diam.) I wonder how that would be(pretty good off road and on acceleration I would think)....but on balance on a DD 50 to 60 MPH secondary roads??? Anyone out there with that tire size and gearing,I'd like to hear what you think performance fuel eco. etc.?????

Great discussion by the way...some good info coming forth..keep 'er coming!!
 

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I agree with this. I think the main reason dealers don't re-gear is because you can not "see" gearing. It would add to the cost something that someone that does not understand gearing/performance might be reluctant to pay for.
Yup, gears aren't sexy, and don't bring in customers off the street.

I would never buy a dealer modified Jeep, they all seem to half ass it and skip the gears. Heaven forbid they spend the extra 800 bucks to get the MAX TOW and do it right. 9or at least better than 3.21's, anyway.

It also makes me laugh at how much they jack the price up on these modded vehicles, at that point you're at or beyond Rubicon money.
 

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This chart should be helpful.

4.10 is what comes on a stock Rubicon (standard on manual, option on auto) with 32 inch tires. I would move up to 4.56 for larger than stock.
 

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I was just talking to the place that would do my re-gear.

He told me that they do 4:56 gears for 33 up to 35 tires and 4:88 for 37's and lower gearing if you request,but for optimal gearing this is what they recommend-do. So I guess 4:56 for my KO2's @ 34X10.5 (33.2 actual diam.) I wonder how that would be(pretty good off road and on acceleration I would think)....but on balance on a DD 50 to 60 MPH secondary roads??? Anyone out there with that tire size and gearing,I'd like to hear what you think performance fuel eco. etc.?????

Great discussion by the way...some good info coming forth..keep 'er coming!!
With 34s and 4.56, that would put you around 2619rpm at 70. With my 37s and 5.13, I run around 2708rpm. A stock Rubicon 4.10 runs around 2502rpm. Those numbers seem high if you are used to older engines, but those numbers upwards 2500+ are where the 3.6 just starts getting into its "happy zone". You have full overdrive, engine and trans are operating most efficiently, you have real power on hills and accelerating and passing when you need pedal to metal.

My Jeep is my DD, and just as at home pulling 75 on the highway getting to the trail or work as it is Offroad rock crawling.

Now of course mpg will take a small hit. But it's a Wrangler - another 1mpg less from something already getting crappy mileage isn't anything. If you are fuel conscious, the JK is the wrong vehicle for you. But also consider you are saving wear and tear on the engine and trans being properly geared - which are not cheap components.

Go to a dealer and test drive a Rubicon with 4.10 bone stock tires. Or hookup with a local Jeep club - folks with properly geared rigs will usually gladly let you take a short test drive to compare for your build. Regearing will feel like a whole new Jeep, guaranteed.
 

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OP;

I own a '14 JK Sport S/auto/3.21 gears. Changed to 295/70r17 Trail Grapplers and changed tire diameter to 33" via AEV Procal. Results from the Procal flash were good enough that I didn't see the need for a gear change. I'm a flatlander as well, so everyday driving wasn't an issue. The only time I consciously thought of re-gearing was when I had to climb the Rockies/Tetons pulling our 1,200 lb. trailer.

Flash forward 13 months. Reluctantly decided to re-gear to 4.11's when adding Truetrac LSD. My expectations were that I wouldn't notice much seat of the pants difference, especially in Iowa terrain, but I'd get the engine power band back closer to where it's comfortable and notice the difference when in the mountains.

The difference has been more than noticeable even in everyday driving at home, the engine downshifts much less often. The unexpected benefit has been a 2+ MPG increase after re-gearing.

If I'd have known then what I know now, I'd have re-geared immediately after adding the 33's. If your tires truly measure 34" mounted and on the Jeep, you'll have more significant results than I have.

Make no mistake, it still has the aerodynamics of a shoebox, still powered by a hamster on a wheel. But he had Cheerios for breakfast this morning and is feeling them.
 

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With 34s and 4.56, that would put you around 2619rpm at 70. With my 37s and 5.13, I run around 2708rpm. A stock Rubicon 4.10 runs around 2502rpm. Those numbers seem high if you are used to older engines, but those numbers upwards 2500+ are where the 3.6 just starts getting into its "happy zone". You have full overdrive, engine and trans are operating most efficiently, you have real power on hills and accelerating and passing when you need pedal to metal.

My Jeep is my DD, and just as at home pulling 75 on the highway getting to the trail or work as it is Offroad rock crawling.

Now of course mpg will take a small hit. But it's a Wrangler - another 1mpg less from something already getting crappy mileage isn't anything. If you are fuel conscious, the JK is the wrong vehicle for you. But also consider you are saving wear and tear on the engine and trans being properly geared - which are not cheap components.

Go to a dealer and test drive a Rubicon with 4.10 bone stock tires. Or hookup with a local Jeep club - folks with properly geared rigs will usually gladly let you take a short test drive to compare for your build. Regearing will feel like a whole new Jeep, guaranteed.
Good info,and yeah I've looked at that gear chart and wondered how that translated to DD duty..I'm not that fuel conscious,it's just that I didn't want to take a huge MPG hit, 1-MPG isn't bad,heck it may even improve my MPG,as the highways I take are all 80 to 90 KPH 50 to 55-60 MPH,and I'm easy on the gas on the highway. I know I'll love the acceleration improvement and off road torque-pull,with the 456 gears.
 

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You will tear up your drive train without compensating for tires. Imagine driving around with your emergency brake on, that is how your power train feels. That tire size and width, 4.11 gearing minimum, and a speedometer gear or program change. If you ran a 32" tire you could squeak by but 34" plus will do some damage. Do not be surprised when your differential gears blow on you.
 

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You will tear up your drive train without compensating for tires. Imagine driving around with your emergency brake on, that is how your power train feels. That tire size and width, 4.11 gearing minimum, and a speedometer gear or program change. If you ran a 32" tire you could squeak by but 34" plus will do some damage. Do not be surprised when your differential gears blow on you.
Why do people make posts like this? This is completely not true.

Is 34's and 3.21s ideal? No, but your Jeep isn't going to explode into a fireball throwing ring gears and pinions into a school bus full of innocent children. The Jeep with 3.21s is slow from the factory. You're probably used to that by now. It's now going to be a bit slower, but it will be driveable. If the regear is in the budget go for it. If not, enjoy your slow Jeep as you already are.
 

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I have to agree that especially if you're on 3.21's to look at re-gearing at least to the 4.10. I'm on 3.73's with 285/75r17 which is basically a 34 at 33.8 on paper and I've definitely noticed the hit in the last few weeks they've been on. It's not horrible, but I can't imagine driving around on 3.21's which were really intended for the base Sport with 29" tires.

Once I'm paid off next March a regear to 4.56 is very likely on my list of stuff to do by the end of next year. Some of the calculators I've used have said 4.0 to restore the factory 32 & 3.73 feel so the closest would be 4.10, but I've also been told it's not worth it unless going up two gear sizes and while the factory setup was ok, it definitely seems like a higher gear than 4.10's would be better since I'm in the manual transmission.
 

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Yup, gears aren't sexy, and don't bring in customers off the street.

I would never buy a dealer modified Jeep, they all seem to half ass it and skip the gears. Heaven forbid they spend the extra 800 bucks to get the MAX TOW and do it right. 9or at least better than 3.21's, anyway.

It also makes me laugh at how much they jack the price up on these modded vehicles, at that point you're at or beyond Rubicon money.
When I bought my jeep the dealer knew I had zero interest in 3.21 gearing and found a 3.73 with max tow. I would have preferred 4.10s but I wasn't going to pay for a rubi. Have not regeared, but agree with others that 4.56 would be the proper gearing with my 34s.
 
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