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Advice on Gear Ratio & tire size for a YJ 4cyl

Discussion in 'General Jeep Discussion' started by GUARDDOGG, Apr 23, 2017.

  1. Apr 23, 2017 at 3:43 PM
    #1
    GUARDDOGG

    GUARDDOGG [OP] Member

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    Hey guys, this is my first Jeep so be gentle. i got a good deal on a 1994 YJ with 31" tires & 4.11 gears so I tore it apart & im completely rebuilding it from the frame up, replacing what I want & need & adding goodies along the way like a RC 4 1/2 lift etc. etc. but my current question is what gears should I run if I have a D35 rear & D30 up front & like the looks of the 33" tires W/ the 4 1/2 lift but need to use this thing at least once a week to take the interstate 45 miles to work and still be able to hit some trails from time to time. Help me oh Jedi of the Jeeps.

    Oh & im YouTubeing the entire build.
    https://www.youtube.com/channel/UCRWai9AYCwQO2C7YKt5f33w
     
    chris4x4 likes this.
  2. Apr 23, 2017 at 8:04 PM
    #2
    OFFGRID

    OFFGRID Well-Known Member

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    Howell fuel injection, header, HEI distributor, TH350, NP208, SOA lift with YJ springs in the front & GW springs in the Rear. AMC20 with G2 1 piece chromos trussed, Dana 30 with G2 chromos and 760x ujoints and MM Stainless Hubs, Geared 4.56. Tom Woods shafts, Metal cloaks, Caged, 37" Toyo MTs.
    First you say you have 31s, but I think that your jeep may have come with 29 inch tires and the 4.11 gears from the factory.

    Formula to change gear ratio in order to maintain stock performance when changing tires size.

    New tire height X old gear ratio (factory) / old tire height (factory)= new gear ratio

    33 X 4.11 / 29 = 4.68

    Round to the nearest gear in this case it is 4.56, but being a 4cyl I would strongly consider bumping up to the 4.88. This also will help if you decide to go 35s which will fit on your setup if you run those flat fenders pictured.
     
  3. Apr 23, 2017 at 8:13 PM
    #3
    GUARDDOGG

    GUARDDOGG [OP] Member

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    Cool thanks, ya I bought it used from a kid and it has 31" tires on it now. I should have all 4 tires on the chassis by tomorrow.
     
  4. Aug 6, 2017 at 3:16 PM
    #4
    GUARDDOGG

    GUARDDOGG [OP] Member

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    Hey OG, I'm getting really close to having the jeep on the road. I was wondering what you think I would be able to do on the interstate if I re gear to 4.88 in the rear with my set up & do I need to re gear the front with the same as the rear?

    IMG_9937.jpg
     
    JKBob 25 and chris4x4 like this.
  5. Aug 6, 2017 at 3:41 PM
    #5
    aggrex

    aggrex Well-Known Member

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    Watching this interesting build! Both axles should be geared 4.88 if you're going to engage 4WD.
     
  6. Aug 6, 2017 at 6:28 PM
    #6
    OFFGRID

    OFFGRID Well-Known Member

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    Howell fuel injection, header, HEI distributor, TH350, NP208, SOA lift with YJ springs in the front & GW springs in the Rear. AMC20 with G2 1 piece chromos trussed, Dana 30 with G2 chromos and 760x ujoints and MM Stainless Hubs, Geared 4.56. Tom Woods shafts, Metal cloaks, Caged, 37" Toyo MTs.
    What Aggrex said. Reasoning. If front axle is turning at a different rate than rear axle you will tear up the T-case. Both axles have to be the same if you plan on engaging 4wheel drive.
     
    GUARDDOGG [OP], aggrex and JKBob 25 like this.
  7. Aug 6, 2017 at 6:31 PM
    #7
    OFFGRID

    OFFGRID Well-Known Member

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    Vehicle:
    1979 Jeep CJ7, 258, TH350, NP208
    Howell fuel injection, header, HEI distributor, TH350, NP208, SOA lift with YJ springs in the front & GW springs in the Rear. AMC20 with G2 1 piece chromos trussed, Dana 30 with G2 chromos and 760x ujoints and MM Stainless Hubs, Geared 4.56. Tom Woods shafts, Metal cloaks, Caged, 37" Toyo MTs.
    I guess I didn't answer you real question. With 4.88s you should be able to drive with 33s-35s on the highway as you were driving with the 31s on your current setup.
     
    aggrex, GUARDDOGG [OP] and JKBob 25 like this.
  8. Aug 7, 2017 at 6:54 AM
    #8
    GUARDDOGG

    GUARDDOGG [OP] Member

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    1994 YJ
    Ground up rebuilding
    Thanks guys, I really appreciate being able to ask elementary questions but this is my first Jeep. It's nice to have a place to go & get help thanks guys.
    I guess I'm going to get 4.88s for both.
    The "real" or justifiable reason for this Jeep is on snow days etc. I have to go to work no matter what so I need something to get 50 miles down the snow covered state & interstate. With that in mind I want to get down the highway in summer to every once in a while so 4.88s worry me a bit with being able to get to 70mph. I don't want to blow this motor up reving it too high for an hour straight.
    I have a spartan locker I'm planning on installing in the rear when I start the re-gear. I'm working to get this thing inspected first though. Thanks guys
    Got the 33s on last night.

    IMG_9958.jpg
     
    JKBob 25 and aggrex like this.
  9. Aug 7, 2017 at 8:03 AM
    #9
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine Moderator

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    High continuous revs isn't going to harm the engine, unless you are over reving the engine.
     
    JKBob 25 likes this.
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