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  • Gearing

    Hello, I have never done any sort of gearing to any of my jeeps so this is new to me, I have an 85 Cj7 258 with D30 front AMC 20 rear with 2.73 gears and the TF999 trans and obviously no power. I was wondering with the automatic what the best gear ratio for me to jump to would be, Im running 31s and may jump to 33s but not for a while. Also do I need to change the gears in both the front and rear axles? I realize Im going to need a new carrier for new gears just wondering what the best gears would be for light offroad, and being able to make it up hills for once while driving on the highway, I also do sometimes pull a small light weight trailer, along with this does anyone have a link to check out for parts to a TF999, mine has recently decided to start slipping from Drive to 2 while going down the road, thanks for the help.

  • #2
    If you are ever going to put it into 4wd you absolutely need both front and rear matching. Some guys will just do the rear and pull their front drive shaft until they can get the $$ to do the front.

    The gears depend on when you're going to go bigger on the tires. If you're sticking with the 31's for awhile you're probably going to be safe in the 4.10 range...This should still give you good driveability and some off road gearing. If you're jumping to the 33's anytime soon you may want to look at a deeper gear in the neighborhood of 4.56. Anything bigger than 33's and you'll definitely want to go to 4.88 range.

    You could do the 4.56 with your 31's but will experience higher RPM's at highway speeds...probably 3K + at 65 or so

    There is a sweet spot between economy and power...if that's what you're looking for you could probably run 4.27...these would have you in the higher RPM range on 31's and little lower on 33's but would work as an economy daily driver gear. You might have trouble finding a set however...I don't believe I've ever seen them for a Jeep, but then I've never looked specifically for them.

    Ultimately gears are a personal choice...only you know how you drive your jeep and what you want out of it.


    • #3
      i would go with 3.73 gears front and rear if your going to stick with the 31's...for the 33's and some light towing i would go with a 4.10 setup...the problem with going to low (numerically higher) is that the pinion gear becomes very small when you go over 4.10's...its not such a big deal on a dana 60 or 9-inch but a smallish axle like a dana 30 develops an extreme weak point when the pinion gets down in the 4.56 and even the 4.27 range

      as far as the slipping problem your having you can probably swap in a new tranny for less than a rebuild on your current 999, unless you do it that case you may need as little as a few clutch packs replaced and your good to go


      • #4
        Check these out:


        • #5
          So heres my next question, most info on here is about manual transmissions and the rpms for how they run, but given that I have an automatic how much higher are my rpms going to be with the same gear ratios. So far Im thinking the 4.10s and I really dont want to hurt my engine but the extra power is intriguing will the 4.10 be to much? 80% of the time I probably wouldnt be driving above 60 and if I do go up the highway Id be happy with 70 as of right now Im lucky to get 65 down hill. Also The Captain mentions a new tranny what would this cost compared to a rebuild on average? Thanks for the help so far its really helping me understand what I need to be happy with this jeep and sorry about not searching other posts but direct answers are so much easier.