My 08 Auto, Sahara has the stock 3.21's in it and I plan on going to 5.13's with 35's. The Saharas and the X's all came with 3.21 gears stock with a 3.73 option and the Sahara had the option of 4.10's I believe. there is another thread that goes more into detail about the gears which to use for which application just not sure what the title was so I am of no help other than you can do a search for it..

 

I would be surprised if you could get your gears done for a few hundred bucks in labor. Here in Alberta (Canada's truck capital) every shop i have been to has told me the same thing. A few hundred for the gears and about 14 hours of labor per axle. So far i have asked 4 different shops. One shop told me they don't do gears because of the difficulty. All shops told me i would be better off buying new axles with the gearing i'm looking for.
Thats just my story.

 

I'm not saying I'm right but from what I've read here and if you goto Jeep.com and build an 08 X or Sahara and Switch from 6spd to auto it changes the gear ratio from 3.21 to 3.73. Check your window sticker.

Also 2008 you can ONLY get the 4.10's in the rubicon, the 07's had 4.10's with the tow package.

 

I just checked the sticker. I have the S package that includes a 3.73 upgrade which begs the question - will I even see any improvement by installing 410's? Aside from being passed out of the hole by Grandma in the minivan my biggest complaint is the difficulty towing my boat and popup camper.

 

Since you actually have 3.73's going to 4.10's would be a total waste of money. 1200-1300 for a miniscule change. Since you are going to tow you want a gear ratio that will put you in the 2400-2700 rpm range and I would bias that towards 2600-2700. With my 35's and 5.13's I'm turning 2600 going down the freeway at 75 mph. With my 35's and 4.10's it was 2050 and stock Rubi with 32's and 4.10's was 2250 rpm (all speeds by GPS). Without running the numbers my guess is that if you want a meaningful increase in performance you need to skip over the 4.56 ratio and go to 4.88. But then as I stated before I think the stock Rubi is improperly geared and should have been speced out with 4.56 gears; that would have put the rpm at 2478.

Using the gear ration calculator at http://www.4lo.com/calc/gearratio.htm
I come up with the following (rpms at 75 mph) with 32" tires: 4.56 is 2478, 4.88 is 2651. For 33" tires: 4.56 is 2403. 4.88 is 2571. For 34" tires: 4.56 is 2332, 4.88 is 2496, 5.13 is 2623.

For your 32 or 33" tires 4.88's are the sweet spot. And THAT would be a meaningful change worth spending the dollars on.

 

I agree w/ the 4.56. Any more and your mpg will suffer.

Here is a chart to help you determine where you are and where you want to be.
Find your RPM at 65mph w/ your gear and plug it into the chart.
Its not exact but it'll get you close and a good idea of how much RPM difference there will be.

 

Just finished reading the whole thread. Glad it's sorted out that you can't get the 3.21's with an auto. 3.21's were only available with the 6spd.

As far as what you should regear to. I think it was said, but if you're not plannin' on going over 35's, towing, driving at higher altitude, then go with 4.88's. If any (or more than one) of those are true, then you might want to look at 5.13's.

 

The problem with the chart is that it only works if the auto trans is in 3rd gear and not in OD. The JK auto has a .69 Ovrdrive ration which knocks the rpm way down. Since the OP is conerned about towing to 4.56 only puts him 200 rpm higher than a stock Rubi (32/4.10). The 4.88 is another couple of hundred beyond that. I would agree that mileage would probably be a bit better with the 4.56 than 4.88 - when not towing. When towing it would probably be worse. The 4.88's would be more bang for the buck; is going from 3.73 to 4.56 worth 1200-1300? The OP has to decide what is most important to him.

 

That's some great feedback Chuck45. Thanks. I have one question. With 33" tires at 4.88 you stated that RPM's would be at 2571 - is that in Overdrive?

 

Yes that is in OD. Your auto has a .69 overdrive ratio which makes a huge difference. The old 3 speed autos where top gear was 1:1 could never run a ratio lke this. MY JK with 35" KM2's and 5.13's runs 2600 (very close to the 2571 mentioned above) and I really Think that is the sweet spot on the engine. Now keep in mind that is at 75mph; if you were going slower, say 60 or 65, your rpm would be lower. With 4.10's or 4.56's your rpm would probably we too far below where this engine makes it's torque.

With 33's and 4.56's you would see 1922 at 60 and 2082 at 65.
With 33's and 4.88's you would see 2057 at 60 and 2228 at 65.

There is a lot to think about when mking the choice as to what gear to choose. At it's cost most of us can't afford to make a mistake.

You might want to pay attention to your rpm when towing at various speeds with the OD off. For instance with 33's and your 3.73 stock ratio and OD off (so no .69 OD but 1:1) you would spin 2278 at 60 and 2468 at 65. Of course I don't recall whether or not you already had the 33's.

As a further aside, and to further confuse the issue, with your 33" tires and 5.13 gears and in OD you would see 2162 at 60 and 2347 at 65.

Confused yet?