Do 255/75R17 equipped 6-speed Sport models come with 3.21 or 3.73? - JeepForum.com
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post #1 of 19 Old 12-17-2014, 10:21 PM Thread Starter
Brazos609
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Do 255/75R17 equipped 6-speed Sport models come with 3.21 or 3.73?

I am looking at a 2012 6-speed Sport, I guess it is actually a Sport S, that came with 255/75R17 wheels to replace my 2005 6-speed LJ. The body tub sticker shows it came from the factory with the 255/75R17 wheels. Did this wheel package make require/include the 3.73 gears as standard. I wouldn't think that they would have installed these wheels with 3.21 gears.

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post #2 of 19 Old 12-18-2014, 06:08 AM
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Those are the stock wheels/tires that come with the 23S package. The 23S package is mostly about power doors, power windows, AC, and other basic creature comforts. It also changes out the stock 16 inch steel wheels for alloy wheels. It doesn't affect the gearing the JK has at all. There's a separate option when purchasing the vehicle that allows you to upgrade to the 3.73 gear ratio. You'll need to check the jeep's build sheet for that, get the VIN and you should be able to have a dealer look it up for you. Or you can simply measure it yourself.
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post #3 of 19 Old 12-18-2014, 07:39 AM Thread Starter
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The JK is 200 miles away so I didn't really want to waste the 6-7 hour round trip to go look at it if it has 3.21 gears. My 05 has the 6-speed, and I assume they both have the same gear ratios, with 3.73 gears in the axles and 255/75R17 BFG MT. I couldn't imagine driving it with those wheels and 3.21 gears in the axles. The 3.21 gears are 16% higher than 3.73 gears. My current highway cruising speed at 75MPH has the engine turning just over 2500RPM. The 3.21 gears would drop the engine speed to 2100RPM at 75MPH and I wouldn't think you could hold that speed at 2100RPM. This JK doesn't have power windows or locks, just the 255/75R17 wheels, A/C, and a hard top. Those appear to be the only factory options.
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post #4 of 19 Old 12-18-2014, 09:17 AM
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Contact the owner and have him send you the VIN, then submit it to a dealer to get the build sheet if you're really curious. FWIW, I got my 2013JKU with 3.73's and a six speed, and I do a lot of highway driving (40K since Jan 2013!) at 75-80MPH. On level ground, you'd maintain speed just fine. I'm running 34's on the stock 3.73 gearing, so I'm running about the same RPMS as you would on 32's with the 3.21. Do I have to shift on hills, obviously, but for cruising I'm just fine.

There's a chart in the JK FAQ that compares RPM's vs horsepower RPM vs. torque , the nice thing about the pentastar is that the torque peak is fairly linear from 2000-5000K. It's got a fair bit of power.
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post #5 of 19 Old 12-18-2014, 09:48 AM Thread Starter
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I just got the VIN checked and printout shows 3.21 gears. I'm just worried about the final drive ratio. From what I could find the gear ratios of the 2005 and 2012 6-speeds are the same except for 6th gear which is 0.797 for the 2012 vs 0.84 for the 2005. With the same wheels I have a final drive ratio of 3.13 in mt 2005 and the 2012 final drive ratio will be 2.56. With the addition 5% higher 6th gear on the JK the difference between my current cruising RPM is almost 20%, about 2550RPM vs 2050RPM at 75MPH. Does the 3.6L have that much more power and is the JK that much more aerodynamic than the LJ to let it hold highway speed at a much lower RPM? The LJ and a 2-door JK are virtually the same weight so that is a wash.
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post #6 of 19 Old 12-18-2014, 09:54 AM
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With stock wheels and tires (255X75X17) the 3.6 has plenty of power. You won't need gears on those tires.
Aerodynamics...LOL.
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post #7 of 19 Old 12-18-2014, 09:59 AM
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I thought I heard somewhere that all the manuals had 3.73

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post #8 of 19 Old 12-18-2014, 11:38 AM
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Quote:
Originally Posted by RILEY-123456789 View Post
I thought I heard somewhere that all the manuals had 3.73
Not true.

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post #9 of 19 Old 12-18-2014, 11:58 AM
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Quote:
Originally Posted by RILEY-123456789 View Post
I thought I heard somewhere that all the manuals had 3.73
Nope. I had to opt to get my 2013 equipped with the 3.73.

Brazos, as I suggested earlier, go look at the dyno curve comparison between the 4.0L and the 3.6L. It's simply a better performing engine in terms of power (won't get into reliability here). @ 2K revs, the 4.0L is putting out ~165HP and ~217ft-lbs. @2K, the 3.6L is putting out 200HP and 250ftlbs. That's a HUGE difference. As I said previously, I run 34" on my JKU with sixth gear on the highway and can easily maintain 75HP.
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post #10 of 19 Old 12-18-2014, 12:31 PM
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Originally Posted by Monkeybomber
Nope. I had to opt to get my 2013 equipped with the 3.73. Brazos, as I suggested earlier, go look at the dyno curve comparison between the 4.0L and the 3.6L. It's simply a better performing engine in terms of power (won't get into reliability here). @ 2K revs, the 4.0L is putting out ~165HP and ~217ft-lbs. @2K, the 3.6L is putting out 200HP and 250ftlbs. That's a HUGE difference. As I said previously, I run 34" on my JKU with sixth gear on the highway and can easily maintain 75HP.
Did they on the older JKs with the 3.8?

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post #11 of 19 Old 12-18-2014, 12:45 PM
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the older 42le autos did come with more gearing. But the needed it as the first gear in that trans is in 2.60 range and the OD(4th) was like .76. So when jeep went to the w580 this auto has about a 3.5 first and .87 od. So what you have is what is 3.21 rear gears in a 2011 would need 4.1.

If you bought a new Rubi auto it comes 3.73 since 2012 as that trans dont need 4.1 to turn 32"s. Now it is a Option though and if anyone is buying a new rubi auto PLEASE do order it with 4.1. You are set for 35"s. stock excepting needed lift and or fender changes etc...I remember picking up our 012 with 4.1 auto on 32"s less mpg but it could smoke those 32"s 2d with no top no rear seat, doo0rs and toss out everything else they are rather quick, for a jeep. for a cj-jk they are the quickest ever made. Jeeps were not made for speed anyway.

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post #12 of 19 Old 12-18-2014, 03:49 PM
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Quote:
Originally Posted by 222Doc
the older 42le autos did come with more gearing. But the needed it as the first gear in that trans is in 2.60 range and the OD(4th) was like .76. So when jeep went to the w580 this auto has about a 3.5 first and .87 od. So what you have is what is 3.21 rear gears in a 2011 would need 4.1. If you bought a new Rubi auto it comes 3.73 since 2012 as that trans dont need 4.1 to turn 32"s. Now it is a Option though and if anyone is buying a new rubi auto PLEASE do order it with 4.1. You are set for 35"s. stock excepting needed lift and or fender changes etc...I remember picking up our 012 with 4.1 auto on 32"s less mpg but it could smoke those 32"s 2d with no top no rear seat, doo0rs and toss out everything else they are rather quick, for a jeep. for a cj-jk they are the quickest ever made. Jeeps were not made for speed anyway.
So what gears would I most likely have in my 09 with the 6 speed manual?

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post #13 of 19 Old 12-18-2014, 09:42 PM
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Unless your planning a regear, hold out for one with 3.73s. You won't regret it.

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post #14 of 19 Old 12-18-2014, 11:55 PM
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riley in on there was a choice of 3:21, 3:73 and 4:10. might be a tag on the rear dif, or you can get a build sheet.
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post #15 of 19 Old 12-19-2014, 08:49 AM
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Originally Posted by pentastarguy
riley in on there was a choice of 3:21, 3:73 and 4:10. might be a tag on the rear dif, or you can get a build sheet.
Thanks!

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