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Unread 03-09-2010, 09:01 AM   #1
WillysGSR
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Re-Gearing

I just put 31ís on and I noticed a drop in power. I use my Jeep as a daily driver and off-road frequently. I was wondering what would be a good gear ratio. Is that something thatís difficult to do your self or would it be best to take it to a shop? Thanks in advance.

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Unread 03-09-2010, 09:22 AM   #2
Wheelin98TJ
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4.56 would work well with your auto tranny and 31" tires.

You would turn about 2,500 RPM at 70 MPH which is just about perfect.

You already have 3.73, so regearing to 4.10 would not be a big enough change to be worth it.

As for the installation, most people leave this job to the pros.
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Unread 03-09-2010, 09:23 AM   #3
admdonitz
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You should take it to a shop, but why would you regear for 31's?
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Unread 03-09-2010, 10:27 AM   #4
MO2500
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Off topic, did you swap in a NV241 from a rubicon?


As for re-gearing, If you don't have an experienced buddy or someone who has done them before to help you on your first gear install i'd recommend leaving it to the professionals. Re-gearing takes special tools, knowledge and a lot of patience to do correctly. Things have to be setup within .001" of spec to ensure you don't mess up your new gears and bearings.

Here's a good thread that been posted recently by Flatlander757 to show what's involved in gear set-up Re-gear thread

Another good article Billavista Gear article
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Unread 03-09-2010, 10:41 AM   #5
dualtjs
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Quote:
Originally Posted by Wheelin98TJ View Post
4.56 would work well with your auto tranny and 31" tires.

You would turn about 2,500 RPM at 70 MPH which is just about perfect.

You already have 3.73, so regearing to 4.10 would not be a big enough change to be worth it.

As for the installation, most people leave this job to the pros.
He's got a 3.07, so a bump to 3.73 would suffice for 31"s (7% jump in tire from 29" off set by a 21% jump in gearing). He could get that via a simple axle swap from a salvage yd (or someone upgrading to different axles).

Not to start an argument, but if he did have 3.73's, 4.56 seems excessive, for 31" tires. I would think that 4.10 would be a more appropriate choice.
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Unread 03-09-2010, 11:14 AM   #6
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With your transmission, go straight to 4.88. No reason not to with the excessively tall .69 OD.
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Unread 03-09-2010, 11:28 AM   #7
Ktjex
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I'm also interested, as I also have an auto with some loss of power after 31s and 3.73 gears. It's not terrible, but definitely could be improved.

So....4.10, 4.56, or 4.88???

What would rpms be like at 70 with the 4.88?
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Unread 03-09-2010, 11:42 AM   #8
Wheelin98TJ
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Quote:
Originally Posted by dualtjs View Post
He's got a 3.07, so a bump to 3.73 would suffice for 31"s (7% jump in tire from 29" off set by a 21% jump in gearing). He could get that via a simple axle swap from a salvage yd (or someone upgrading to different axles).

Not to start an argument, but if he did have 3.73's, 4.56 seems excessive, for 31" tires. I would think that 4.10 would be a more appropriate choice.
I just checked the profile and noticed the 3.07 gear ratio. That is probably incorrect.

All non-Rubicon TJs with the 4.0 and the 4 speed auto are supposed to have 3.73 gears.

4.56 may seem excessive for 31" tires, but not when you consider the OD ratio and the RPM numbers I posted previously.
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Unread 03-09-2010, 11:42 AM   #9
dualtjs
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Quote:
Originally Posted by Ktjex View Post
I'm also interested, as I also have an auto with some loss of power after 31s and 3.73 gears. It's not terrible, but definitely could be improved.

So....4.10, 4.56, or 4.88???

What would rpms be like at 70 with the 4.88?
With 31's & 4.88's you would have to run 2560 rpm to hit 70. With stock 29's and 3.73 gearing, you needed 1950. While I understand the need to deal with the addition weight of the larger tires, that's a HUGE jump.
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Unread 03-09-2010, 11:44 AM   #10
Wheelin98TJ
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Quote:
Originally Posted by Ktjex View Post
I'm also interested, as I also have an auto with some loss of power after 31s and 3.73 gears. It's not terrible, but definitely could be improved.

So....4.10, 4.56, or 4.88???

What would rpms be like at 70 with the 4.88?
3.73 to 4.10 is not a big enough change to bother with.

4.88 will turn about 200 more RPM at 70 vs. 4.56, so you would be around 2,700 RPM at 70 MPH with 31" tires.
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Unread 03-09-2010, 12:05 PM   #11
dualtjs
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Originally Posted by Wheelin98TJ View Post
3.73 to 4.10 is not a big enough change to bother with.
Unless all he wants to do is gain back the drop off from going to 31's (i.e, basically zero intention of going taller on the tire) AND can find a cheap set of 4.10's out a 4 cyl. They would be straight bolt in for minimal cost and he could then sell his 3.73's for close to the cost of the 4.10's. There are a lot if "if's" in that statement, but if he got lucky, he could do the install himself, for something in the neighborhood of the cost of gears for 1 axle.
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Unread 03-09-2010, 12:20 PM   #12
iawia
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I'm very interested in this too as I want to change to either 30's or 31's, preferably the latter, but I'm concerned about power loss as the OP mentioned.

I have a stock 4.0l TJ which does come with 3.07 gears so an increase to 3.73 or 4.10 would seem to be enough. The 4.10 from a 4 cyl seems like a good idea and likely easier to find than the 3.73, although that's what I have in my truck and would prefer.

This question has answered a question I was just about to ask in a new post. I know 31's will fit in a stock TJ, maybe with a bit of rub, but are they too big for the 3.07 axle? The answer seems to be yes.

I hope the OP won't mind if i ask a related question which is probably now moot: Will 10.5 tires fit on the stock 15x7 rims or are 15x8's needed?

If you regear, don't you have to do both front and rear axles?
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Unread 03-09-2010, 12:23 PM   #13
iawia
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I forgot to mention in my previous post that there is a thread here on doing a regear by yourself.

http://www.jeepforum.com/forum/f9/ex...thread-965764/
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Unread 03-09-2010, 12:56 PM   #14
dualtjs
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Quote:
Originally Posted by iawia View Post
I'm very interested in this too as I want to change to either 30's or 31's, preferably the latter, but I'm concerned about power loss as the OP mentioned.

I have a stock 4.0l TJ which does come with 3.07 gears so an increase to 3.73 or 4.10 would seem to be enough. The 4.10 from a 4 cyl seems like a good idea and likely easier to find than the 3.73, although that's what I have in my truck and would prefer.

This question has answered a question I was just about to ask in a new post. I know 31's will fit in a stock TJ, maybe with a bit of rub, but are they too big for the 3.07 axle? The answer seems to be yes.

I hope the OP won't mind if i ask a related question which is probably now moot: Will 10.5 tires fit on the stock 15x7 rims or are 15x8's needed?

If you regear, don't you have to do both front and rear axles?
31X10.50's will fit stock wheels without a prob. You'll probably need a washer or two on the steering stop, but it's unlikely that you will notice a difference in steering radius.

I have a manual, so performance with auto would be a little different. There is a noticeable change in the performance, from a power perspective, but I can also say that I saw a jump in mpg (I'll be giving it back when I do an axle swap, because I want a little more power performance when wheelin'). As far as wheeling is concerned, I haven't done any hard core hill or rock climbing and haven't had any probs keeping up when trail riding. I performance could be better, from an available power perspective and I end up riding the clutch more than I'd like. If I were staying at 31", I would probably just buy some used axles with 3.73's and put them in. I'll buy some 4.10's when I go to 33's. 4.56 would provide more wheelin power, but I can't afford to have to run the rpm to 2,600 to do 70 on Atlanta freeways (around here, you'd get run over if you went any slower).
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Unread 03-09-2010, 01:24 PM   #15
iawia
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Thanks, dualtjs. I remember the freeways around Atlanta when I did some consulting in Kennesaw years ago. Also got a chance to see red Georgia mud.
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