Jeep JK Automatic with 285/70r17 - 33's - PERFECT GEARING Choice

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  1. #1
    JK Newbie Cats&Jeeps's Avatar
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    Default Jeep JK Automatic with 285/70r17 - 33's - PERFECT GEARING Choice

    Greetings all...

    I would like to thank every one that has posted ring & pinion gearing options on this site. It has been a tremendous help in selecting the perfect gear set-up for my needs. I will however say this, more information needs to be addressed when suggesting gear options for the JK automatic.

    Back info.... I have an Unlimited with Auto and came stock with 3.73's and 255/75r17 32" tires. I upgrated to 33's, well 32.8's with BFG KM2's 285/70r17's. Since buy the unlimited auto site unseen without driving the auto, I have been very disappointed with the power to weight ratio and drivability of the auto and 3.8l motor combo. Issues like, will not hold speed on e-way, will not climb slopes or hills without down shifting and would not pull a small 800lbs trailer through the mountains on factory tires and gearing, eventhough it had the "trailer" package

    Additional research and info before I offer a tremendous suggestion for gearing options... I owned a 2002 Jeep liberty, had the 3.7L and 45RFE Automatic Transmission - it was the first and last year the Liberty had this transmission, it soon went to our JK beloved 42RLE. I loved this jeep, it ran hard and cruised the highway well adn had pretty goo pep for a V6. Pulled a u-haul trailer with no issues at all. Keep this in mind as you read later....

    Here is the connection to the JK issues mentioned above.... lets let logic preveil here as well as seat of the pants feel....
    1. The 02 Liberty and 45RFE Automatic Transmission has a .75 OD and 3.0 first gear with 27" tires and 3.73 gear ratio.

    2. The JK auto 42RLE has a .64 OD and 2.84 1st gear with 32"tires and 3.73 gears

    OK, it does not take a Jeep engineer to see that we are going to have issues here with these comparable set-ups.... think about this.... the transmisison in the JK will have to work a lot harder to get down the road... oh and by the way (the dodge 42 auto was used with the 4cyl Jeep Liberty's - is it weeker than the 45 autos?) I cna see why Chrysler decided to go with a transmision with a .64 OD... fuel milleage. But it has hurt the Jeep Auto line with this transmissions... look at other boards and you will see the same issues with current Liberty's adn others.

    Ok, here is the math... I wanted 02 Liberty performance out of my JK... why, well they have very similar motors, the vehicle wgt is about the same and yes the shape is different, but I'm not drag racing so this is a non-issue... lets look at the transmission OD, axel gear ratio and tire matching (final drive ratio) to find the correct gear ratio for my JK auto.

    Premis 1. The final drive ratio of the Liberty with 27" tires is: .75 x 3.73 = 2.7975
    Premis 2. The Final drive ratio of the JK with stock 32" tires is .64 x 3.73 = 2.3872
    Premise 3. The 32" tire is 1.1852 larger than the 27" tire.
    Conclusion 1 - if I want Liberty performance on y JK with stock tires I would of needed a final drive ratio of 2.7975 with 27" tires, but I have 32" tires so I need to factor the larger tires into consideration.... 2.7975/.64 = 4.37 (4.10 - options on 07's only with tow package - non rubicons)
    Conclusion 2 - Now, Considering the larger tires multiply 1.1852 x 4.37 = 5.18 to adjust for larger tires, so my conclusion is that to have 02 Jeep Liberty performace on my "Stock" Jeep JK with automatic, I needed 5.13's from the factory to make the performace the same as my beloved 2002 Jeep liberty with same motor and better transmission

    Now, lets consider a tire change to 33's, how will that effect the performace... well as we all know... to get the performace back to stock with a tire size increase, we will need to re-gear.....

    Premise 1. Consider the "should of" gearing of 5.13's for stock tires as proved above.
    Premise 2. Change of tire size from 32's to 33's... 33's are 1.03125 larger.
    Conclusion: Multiply 1.03125 x 5.18's = 5.3425 (5.38's). 5.38's is what i should use with 33 inch tire.

    Well, I know many of you will chime in and say your gas milleage will decrese alot and the engine will over rev.... well these 3.8L need higher rpms to pull hills and I bet any of you that know transmisisons will agree, that these ideas will reduce the drag on the tranny and its willingness to shift too often and over heat.... i.e. potential of fires.

    Final conclusion....

    I first went with 4.88's auto and 33's, tried to pull my trailer through the mountains in Glacier Park.... it was bad, 35mph in 2nd gear with the newly installed trans warning going off.... I wanted a diesel then and know.... if I could of bought a JK diesel then, I woudl of gladly parked my gas JK with a free sign on it

    So, I regeared 5.38's with 33's auto trans and pulling an 800lbs trailer.... WOW!!! I like my JK again, it pulled hard on small mid-western hills with very little upshift out of OD.... from Minneapolis to Marquette, it upshifted 3-times pulling at 65mph and 2,400 rpms. I had 16 miles to the gallon on the round trip, verified by actual miles and gallons. The JK is fun to drive again. Without pulling the trailer, the JK is a true 4-speed transmission with 18-19 miles per gallon. Wind greatly affects my mileage... 3' lift.

    There was a post on here before that said a more efficient motor will use less gas... well, I believe that tremendously.... when re-gearing.... go over the top, you just may like it!

    ***** Correction's made from original post.... math was incorrect, JK's are .64 OD's not .69..... sorry for the confusion**** the 5.38's are still WOW-ers.... with 33's and auto...

    THANKS for Reading.... please offer your thoughts and experiences.... cheers
    Last edited by Cats&Jeeps; 07-09-2011 at 09:27 PM. Reason: incorrect JK OD.... .64

  2. #2
    JK Enthusiast chipmaker's Avatar
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    Does it wind up at 75-80 mph?
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  3. #3
    JK Super Freak CerOf's Avatar
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    I was like you but with my old ZJ grand Cherokee. People thought I was crazy to run 4.56 gears with 31" tires.

    In town mileage went up by 2mpg and hwy dropped about 1, but was much more enjoyable to drive. Had I gone to a 32" or 33" I believe my highway would have gone up by at most .5 and in town wold have stayed better or at least stock.

    This is of course to counter act the lift, ARB bumper, winch, rock rails, tire carrier, etc.

    Gears just make sense when you add weight or go up in tire size.

    The 3.8 likes to rev, so let it rev with more gear.
    '09 Rubicon Unlimited
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    '02 WJ 3" OME/spacer lift and lots of other stuff.(rip)
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  4. #4
    JK Newbie Cats&Jeeps's Avatar
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    Quote Originally Posted by chipmaker View Post
    Does it wind up at 75-80 mph?
    I usually do not run it more than 70mph on the e-way without the trailer. Gas Mileage will go down to about 16 at 70-74mph with no winds. My wife likes to run it at 75 when she drives. at 70mph, it pulls at 2600rpms. 2300-2600 feels confortable for the JK. My 02 Liberty would pull 2300 rpms on the e-eway at 75mph and still get 19mpg, 23mpg at 65. I think the 02 Liberty set-up was spot-on by Chrysler, its sad they made the change in future Liberty's and set-up the JK the way they did.... just gives them both poor performance.

    FYI - MY JK factory RPM's with stock tires and 3.73 seem to pull real low "almost lugging" rpms 1600-1800... My 94 YJ, pulled those RPMS but had the torque to make it happen with the 4.0L... that was the right motor for Jeeps....

    If you-all consider the "Go-Devil" motor in the first Willy's... 60-some horses on a 2k lbs jeep with 700-16's; 29" tires?.... they made it work with 4.88's and 5.13's with a final ratio of 1:1 tranny... so it all makes sense to go bigger gear ratio with the JK Auto's. It just irritates me that I first went with 4.88's instead just taking the plunge with 5.38's the first time. My wife always says that I fix things twice before getting it right

  5. #5
    JK Enthusiast OscarTheJeep's Avatar
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    Cats&Jeeps, My previous Jeep was an '03 KJ with a 45RFE. I couldn't agree more with you! It was perfectly geared from the factory. Now that I've got 33's on my JK, i'm less than impressed by it's performance, even with 4:10's. It's livable, but it's not like my Liberty was.
    '09 2dr JK - Stock at the moment

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