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Old 06-06-2012, 05:12 PM   #1
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What is the problem with 3.21 gears

What is the problem with this gear set up. Sorry I don't understand. Looking at used jeeps - how would I know be able to confirm? See people talking about the 3.73. What is the difference?

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Old 06-06-2012, 05:20 PM   #2
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Google "axle gear ratio".
And there is nothin wrong with 3.21 if you dont tow anything and plan on sticking with the factory size tires

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Old 06-06-2012, 05:21 PM   #3
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If you are looking at 07-11 auto...avoid them.

If you are looking at a 2012-13 (and plan on keeping it stock and not towing)....you will be OK for simple around town driving....especially on a 2 door.

It is all about getting the engine in the right RPM range. The 3.21's were an awful match for the old 4 speed auto tranny combined with the 3.8L.

The final drive ratios were just all jacked up. That is why people called the 3.8 a dog, but it was never really the 3.8 that was all that bad, it was the gearing of the tranny and the axle that made it look really bad.
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Old 06-06-2012, 05:21 PM   #4
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Numerically higher gears mean that you'll run higher RPMs at the same speed.

So for example, whereas you might be at 2000 RPMs at some particular speed, an identical jeep with 3.73s might be at 2200 RPMs at that same speed. The way engines work is that as RPMs rise, so does horsepower. So deeper gearing (numerically higher) means that you have more horsepower on the ground at a given time. For similar reasons, it can handle bigger tires. It's more complicated than that, but that's the idea.

But don't worry about it. Just testdrive jeeps and see what you like.
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Old 06-06-2012, 05:25 PM   #5
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Thanks,
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Old 06-06-2012, 06:03 PM   #6
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I had test driven an unlimited with 3.21 and one with 3.73. To me the 3.21 accelerated just fine with a smooth transition between gears. The 3.73 did the same with more punch to it. I like the feel of the 3.73 better. Should I need to tow something........I would like to have the capacity to do so with the higher weight limit of 3.73. Mostly the hitch will hold a bike rack for we have a 3/4 ton 'burb to pull our camper. Go test drive .........see which one works best for you.
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Old 06-06-2012, 06:24 PM   #7
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You might want to mention that the lower the gear (higher the number), the lower the gas mileage......
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Old 06-06-2012, 06:41 PM   #8
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I'm very happy with the 3.21 gearing on my 2012 JK Sport two door. Had I been placing a factory order, I would have gone with the 3.73 gears, but I bought off the lot.

I have been pleasantly surprised by the acceleration with the 3.21 gearing. That doggone Pentastar 3.6L engine puts out enough horsepower and torque to make the 3.21 gears work just fine.

I have no plans to go with larger tires or to do any towing, so the 3.21 gears should work out just fine for me over the long run.

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Old 06-06-2012, 06:51 PM   #9
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You can always get the V.I.N. and register at Jeep.com and it will tell you how the used Jeep came equipt as well as dealership service records. I feel I got lucky when I found my 2009 with 3.73's. Good luck to ya.
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Old 06-06-2012, 07:01 PM   #10
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Heh Heh--WRONGO---


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Originally Posted by herebyproxy View Post
You might want to mention that the lower the gear (higher the number), the lower the gas mileage......
The lower the gear ratio, the better opportunity to get the V6--3.8L/3.6L-into there optimum cruising zone and -

Thats gonna be different with each jeep/tires/tranny/driving technique/weight-

My gas mileage went up avg 15/20%, with my regearing--I found the "comfort Zone" for my 3.8L "RACE" engine !!

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Old 06-06-2012, 07:22 PM   #11
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Originally Posted by MTH View Post
Numerically higher gears mean that you'll run higher RPMs at the same speed.

So for example, whereas you might be at 2000 RPMs at some particular speed, an identical jeep with 3.73s might be at 2200 RPMs at that same speed. The way engines work is that as RPMs rise, so does horsepower. So deeper gearing (numerically higher) means that you have more horsepower on the ground at a given time. For similar reasons, it can handle bigger tires. It's more complicated than that, but that's the idea.

But don't worry about it. Just testdrive jeeps and see what you like.
MTH is absolutely right but I respectfully would like to offer an alternate thought.
Given that you're already on the forum, you too may be sucked into the "I must modify it" vortex. I have the 3.21s- I was unconcerned about towing, happy with the HP and figured it wasn't a "do or die" option. I found a great deal on a Jeep that just happened to have 3.21s and bought.
However, now I know that the 35" tires/wheels/lift kit I bought, in concert with my 3.21 gears, might result in a loss of horsepower or MPG. I couldn't care less about the MPG but I do have a lead foot. As MTH said its complicated, and results/opinions vary wildly- there are a gazillion threads. But with 3.73s (at least) you stand a better chance of not being bogged down with modifications you may want later. If you just want your Jeep to be a fun everyday driver and plan to stick to the stock stuff- there is nothing wrong with 3.21s. (or DDs!)
I won't be able to get to my install for a couple or weeks to know how the 3.21s will do. But to put it in prospective- a change out to 4.56 gears has been quoted at $900-$1400. Compare that to the $50 (?) factory option. Think of it as insurance against the vortex.
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Old 06-06-2012, 10:29 PM   #12
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how does the 3.6 with a auto and 3.21's do with 35's? just going with 35's until i have enough saved up to regear
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Old 06-06-2012, 10:29 PM   #13
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You might want to mention that the lower the gear (higher the number), the lower the gas mileage......
That sounds about right but not necessarily true. My 3.21s and my 35s dog the engine really bad in fifth and sixth gears. I stopped shifting past fourth a while back and now I have a jeep that can compete on the highway with the rest of the cars. When I ever regear I'm going with 4:88 and that might be better than anything you can get stock.

Since I stopped shifting past 4th my gas mileage has not suffered. Maybe 1mpg but I have plenty of fun jeeping power.
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Old 06-06-2012, 10:36 PM   #14
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We just purchased out '12 about three weeks ago now, 3.21's, unlimited sport. And I have to say there is more than enough punch with this setup. My wife uses it to commute and we love to take long out of town trips with mild offroading. The better MPG's with the 3.21's was a selling point for us. Just returned from a trip from Phoenix to Vegas, and back with a 4wheeling stop in Sedona on the return. We averaged 23.1 MPG's on the highway cruising around 75mph. Down the road, when it becomes a toy, then the tall lift with 35's and at least 4.11's will be going in. Not sure if this helps, but just my two cents.
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Old 06-06-2012, 10:37 PM   #15
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Quote:
Originally Posted by JeepHerz View Post
Given that you're already on the forum, you too may be sucked into the "I must modify it" vortex.
exactly!!!! if you're here, there is a high probability of being bitten by the "mod" bug.
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Old 06-06-2012, 10:40 PM   #16
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Sounds good, but they'll be 4.10 diff gears, instead of the old standby 4.11 !


Quote:
Originally Posted by Rocjeepn View Post
We just purchased out '12 about three weeks ago now, 3.21's, unlimited sport. And I have to say there is more than enough punch with this setup. My wife uses it to commute and we love to take long out of town trips with mild offroading. The better MPG's with the 3.21's was a selling point for us. Just returned from a trip from Phoenix to Vegas, and back with a 4wheeling stop in Sedona on the return. We averaged 23.1 MPG's on the highway cruising around 75mph. Down the road, when it becomes a toy, then the tall lift with 35's and at least 4.11's will be going in. Not sure if this helps, but just my two cents.
Immaterial-

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Old 06-06-2012, 10:42 PM   #17
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Immaterial???
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Old 06-06-2012, 10:47 PM   #18
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The mathmatical difference between the two and the correction to the semantics--


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Immaterial???
End result, since this isn't a certified tech data document--IMMATERIAL !

Capish ?

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Old 06-06-2012, 10:48 PM   #19
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This thread lacks pictures.. Time to fix that.. Yellow bad.. Be in the green kids, Be in the greeeennn. BUT BUT BUT if your really special in rare cases you are even good in da blue.. BLUE! da da blue daba diba daba dye





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Old 06-06-2012, 10:49 PM   #20
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Quote:
Originally Posted by Rocjeepn View Post
We just purchased out '12 about three weeks ago now, 3.21's, unlimited sport. And I have to say there is more than enough punch with this setup. My wife uses it to commute and we love to take long out of town trips with mild offroading. The better MPG's with the 3.21's was a selling point for us. Just returned from a trip from Phoenix to Vegas, and back with a 4wheeling stop in Sedona on the return. We averaged 23.1 MPG's on the highway cruising around 75mph. Down the road, when it becomes a toy, then the tall lift with 35's and at least 4.11's will be going in. Not sure if this helps, but just my two cents.
The way mine sits right now I would be wore out trying to maintain 75 in 6th gear. 5th gear is doable but actually 75 is a little scarey for me in my jeep. Too short a wheel base although I have driven it that fast but not far.
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Old 06-06-2012, 10:58 PM   #21
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Man I'm in true Blue at 70 mph and "smokin" 2600 rpm

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Old 06-06-2012, 11:30 PM   #22
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OK So I have 3.21 gears in my New 2012 JK 2dr and I plan on running 33's with a Teraflex leveling kit, do I need to re-gear? I will never go rock crawling, but I do intend to do lots of trail and back country driving and even possibly someday trying Black Bear Pass.

An another note how hard is it to change out the gears? I see tons on eBay for what appears to be a good price. I am quite handy with my tools and not afraid to get into the diff and get my hands dirty.

Thanks.
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Old 06-06-2012, 11:52 PM   #23
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1. you will be fine with 3.21's and 33's. Won't be ideal but I wouldn't bother regearing for it.. 2. Regearing is nothing you should ever tackle by yourself unless you are very experienced in diff work.. This is a job that needs to be done by a profesional and a place where a waranty is put on work done.. The slightest screw up and you will be in trouble or at least your wallet will be. It does cost 1200 bucks +/- 300 usually sometimes more but it's a cost well worth it if you ever do regear..
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Old 06-07-2012, 12:19 AM   #24
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Look at the chart above and you'll see there is only a slight change in rpms at 70 with a 33". Up to you to decide if you think you'll even notice. The 35" tire however drops you all the way down to 1700 and change. Ouch. Again, you can ask 5 people and they might have 5 different opinions.
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Old 06-07-2012, 01:20 AM   #25
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Quote:
Originally Posted by dprimm
OK So I have 3.21 gears in my New 2012 JK 2dr and I plan on running 33's with a Teraflex leveling kit, do I need to re-gear? I will never go rock crawling, but I do intend to do lots of trail and back country driving and even possibly someday trying Black Bear Pass.

An another note how hard is it to change out the gears? I see tons on eBay for what appears to be a good price. I am quite handy with my tools and not afraid to get into the diff and get my hands dirty.

Thanks.
It's personal preference basically with that setup. I had 3.21 gears in my '12 2dr auto and went to 33" Duratracs with a 2.5" TeraFlex spring/shock lift. It was still ok with the 3.21's but I noticed a drop in takeoff power and passing power as well. I'm a bit of a fan of a quick vehicle myself so I re-geared to 4.10's with an Auburn HP LSD. I love it now! It really jumps off the line from a stop and with the power of the 3.6 it is great passing and I am only at 2500 rpm's at 74-75 mph!
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Old 06-07-2012, 02:21 AM   #26
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What is the problem with this gear set up. Sorry I don't understand. Looking at used jeeps - how would I know be able to confirm? See people talking about the 3.73. What is the difference?
Nothing wrong as long as you don't get larger tires and don't tow a lot. And even if you tow and within the limit, you might just a little harder time during that 1% of your jeep time and you're still good for 99% of other time.

And you have better chance to get closer to the claimed gas mileage.

Other people here might tell you otherwise because they lift, rock and tow
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Old 06-07-2012, 05:32 AM   #27
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Wow. Lots of information. Thanks everyone!
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Old 06-07-2012, 07:31 AM   #28
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Immaterial???
I agree that 3.21's on the 2012's are not that bad....especially as a non-modded DD. But keep in mind that the OP is looking at used Jeeps......so we are probably talking about a different tranny AND engine.
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Old 06-07-2012, 08:05 AM   #29
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It's personal preference basically with that setup. I had 3.21 gears in my '12 2dr auto and went to 33" Duratracs with a 2.5" TeraFlex spring/shock lift. It was still ok with the 3.21's but I noticed a drop in takeoff power and passing power as well. I'm a bit of a fan of a quick vehicle myself so I re-geared to 4.10's with an Auburn HP LSD. I love it now! It really jumps off the line from a stop and with the power of the 3.6 it is great passing and I am only at 2500 rpm's at 74-75 mph!
If you don't mind me asking how much did that run you? Where did you get it done? How long did it take? I'm in the Boston area as well.
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Old 06-07-2012, 08:15 AM   #30
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It's personal preference basically with that setup. I had 3.21 gears in my '12 2dr auto and went to 33" Duratracs with a 2.5" TeraFlex spring/shock lift. It was still ok with the 3.21's but I noticed a drop in takeoff power and passing power as well. I'm a bit of a fan of a quick vehicle myself so I re-geared to 4.10's with an Auburn HP LSD. I love it now! It really jumps off the line from a stop and with the power of the 3.6 it is great passing and I am only at 2500 rpm's at 74-75 mph!
I see in your pic you have the stock Jeep wheels, did you need spacers with those new tires?

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