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3.21 to 4.56 Gears

15K views 41 replies 23 participants last post by  weldporn 
#1 ·
I dropped off my 2014 6-spd with 3.21 gears to have them swapped with Yukon 4.56 yesterday. I'm running 35's and it was a complete dog not to mention 6th gear was essentially useless. That is all...I'm just excited to get her back. Thoughts or comments are appreciated.
 
#3 ·
Thinking of doing the exact same thing to my 2010 6 speed that also has 3.21's.
 
#6 ·
Same here. And splurged in a trutrac for the rear at the same time. Love it.
 
#8 ·
they are the gears in the differential of the axle. Those are the limiting factors of your first gear and overdrive. They basically shift the whole drivetrain ratio so that you can get into a better power band for larger tires(which also change your drive ratio)
 
#19 ·
Dont forget to get a flashcal or other type of programmer to let the computer know what its dealing with as far as gears and tire size . it will also make the speedometer read properly . FYI
 
#20 ·
Thanks for the ProCal reminder I had them do that for me before I picked it up.

What a difference with the new gears! It has so much more power and I have 6th gear back. This was long over do an certainly not a sexy purchase but well worth every penny.
 
#22 ·
Picking up my 2012 JKUS auto today. Ordered the OME 2.5 lift and decided on running the duratracs 315/70r17. I do have 3.21 gears though which I will upgrade at a later time to either 4.10 or 4.56. Can some one hold me and tell me I'll be ok with this set up for now...lol 😜
 
#23 ·
Picking up my 2012 JKUS auto today. Ordered the OME 2.5 lift and decided on running the duratracs 315/70r17. I do have 3.21 gears though which I will upgrade at a later time to either 4.10 or 4.56. Can some one hold me and tell me I'll be ok with this set up for now...lol ��
Flat land or hilly highways? (I know texas has both.) Flat land will not be too bad, but hills the auto trans will hunt for the right gear.

I run 33's with 3.21 gears and have hilly terrain, but I have a manual trans so it goes into what gear I want it to. :D

If you are coming from a sports car or a car with lots of power, it will seem like a dog. If you are coming from a 4 cyl short on power but great on gas, you will not see much of a change, well, except for worse on fuel!! :)

Oh - and welcome to the forum!
 
#25 ·
I ran 33's in my auto TJ with 3.07s for waaaaaaaaay too long. I upgraded to a Super 35 with ARB/D30 with chromo shafts and TrueTrac with 4.56s when I went up to 35's and the difference was amazing. It was like a whole new Jeep!

There is a lot to read about the importance of proper gearing but the price makes it hard to swallow for some people. I will always recommend proper gearing for tire size. You're going to be very happy with the new set up! Have fun with it.

-Ryan
 
#27 ·
I'm torn between 4.56 and 4.88 for gears in my 35" tired, 3.8L automatic. I currently have factory 4.10s, but it's always hunting for gears on the freeway. I just can't decide if 4.56s would be worth it, or if I should just go with 4.88s. All of the gear calculators tell me 4.56 is what I should do, and I'm worried that 4.88s will be a little too much for the driving I do.
 
#28 ·
Just got mine back yesterday, 4.88 gear and arb locker front and rear. Added some nice custom painted poison spider diff cover. Can't wait to drive her! I'm away from home for work so she is waiting for me at home.

Still need to do the hook up of lockers as the shop didn't wanted to do it for good reason.

Next is swap my 33" for 35" my 33 are almost done so I'll swap when they are done.
 

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#31 ·
Just also got my Jeep back from the shop. Had the 3.21's removed and swapped in 4.88's. I'm running 33's for the moment until it's all dried up and then throwing on 35's (maybe 37's) when the time comes.
For $2100, I got a re-gear done with Detroit Truetracs (front & rear), with Yukon 4.88's with the master overhaul kit. Still breaking her in but so far I couldn't be more pleased with the power return. I live with lots of hills so the 3.21's were totally insuffucient to my driving needs.
 
#37 ·
And 4.56's in 6th gear @70 mph



View attachment 1639098
Have you adjusted with Procal for new gears?

According to Gear Ratio Calculator it appears that your RPM's are a little higher than they should be.

Thanks for that post Gthoren. Interesting the big difference between an auto and manual. At 70 MPH in an auto with 4.56s I am where you're at in 6th gear- about 2700 RPMs. Which is perfect for me. What tire size?

Just my 2 cents on gears- my Yukon locker kind of fell apart. I had 5 bolts rambling around in my diff case for a couple thousand miles. My G2 gears had barely a nick on them. For that reason I HIGHLY recommend G2 gears.
According to GrimmJeepers gear calculator the auto w/ 4.56's should be turning 2591 RPM, which is closer with JeepHerz comment of ~2700 @ 70 mph.

The manual Jeep with 4.56's should be turning 2466 RPM in 6th gear (based on 315/70/17 tires)

Just trying to sort out the variables...

If you had a dealer reprogram, they cannot adjust tire size larger than 32" So for example if you plug 255/75/17 into the gear ratio calculator it shows 2643 RPM @ 70, which leads me to believe that your not reprogrammed correctly for the new gears and bigger tires.
In reality, you are probably running closer to 75 mph at that RPM. Download a GPS app onto your smartphone and check it that way.

FWIW, JeepHerz. If I plug 255/75/17 in for auto, the RPM's show 2777 at 70 mph


Not tearing anybody down, just really working this whole re gear thing...
 
#35 ·
Thanks for that post Gthoren. Interesting the big difference between an auto and manual. At 70 MPH in an auto with 4.56s I am where you're at in 6th gear- about 2700 RPMs. Which is perfect for me. What tire size?

Just my 2 cents on gears- my Yukon locker kind of fell apart. I had 5 bolts rambling around in my diff case for a couple thousand miles. My G2 gears had barely a nick on them. For that reason I HIGHLY recommend G2 gears.
 
#36 ·
Thanks for that post Gthoren. Interesting the big difference between an auto and manual. At 70 MPH in an auto with 4.56s I am where you're at in 6th gear- about 2700 RPMs. Which is perfect for me. What tire size? Just my 2 cents on gears- my Yukon locker kind of fell apart. I had 5 bolts rambling around in my diff case for a couple thousand miles. My G2 gears had barely a nick on them. For that reason I HIGHLY recommend G2 gears.
I am running 35's.
 
#38 ·
Keep in mind that the torque curve on the Pentastar is flat as a pancake between 2300 and 3000 RPMs. -+ 200 RPMs is not remotely noticeable at highway speeds. We (all) make a big deal over highway/gear ratio but the difference between two close gear ratios is not as big as people think. Yes- 3.21s to 4.56s is a jump- but 4.56s to 4.88s isn't that different because of the relatively small difference in RPM. 33s to 35s made only a small difference. But yes- getting into that 3000+ RPM range is a valid concern. Mini van engines don't sound good at that RPM. LOL
(I am not speaking to MPG because for me it's a non-issue. It sucks).
And Grimms calculator is excellent to use. The gear chart only covers one speed- city driving is also important.
 
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