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Axle Gearing help

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  • Axle Gearing help

    Anyone in here that has run an R2.8 and ax15 in a Jeep CJ or similar lighter weight vehicles with bigger tires? I'd like to run 35s and can't get much clarity on an ideal gear ratio. Currently leaning toward 4.10s but not sure what RPMs are optimal for everyday driving. Everyone I talk to at cummins points to 2000rpm as the magic number but I know there has to be a better sweet spot than that. I'll sacrifice a few mpgs...

  • #2
    4.10s would be about the best of all worlds for 35s. What kind of driving are you doing?

    I'm running ax-15 with 33s and 3.73s should be perfect. I am in the middle of the axle swap, so I haven't got the gears in there, but I've put 27.5" tires which is the equivalent, besides the extra weight and drag from the tires and it's great all around. If you were doing more off-road and backroad driving, 4.10s would be a bit better for 33s and if it was on a trailer going to trails, well I'd go with a really big ratio.

    If you planned on a trailer and you want more torque you may look at a lower ratio, but I've towed with 3.07s and 33s and it's not that bad and most hills I don't have any trouble with until I got a roof rack and then I started having trouble climbing steep hills in 5th. With 4.10s you won't be shifting on the highway from 5th.

    I think of 1800 as a sweet spot for 5th gear, but all other gears 1,500 rpms is great. You just need to have the turbo whistling a bit. If you want a bit more kick in the pants than yeah 2k is great, but you really won't want to go into any hills under 1,800 RPMs with any type of load. Empty and low drag 1,500 RPMs is perfectly fine.


    • #3
      Hey thanks for the reply. It will be almost all highway driving and it's pretty much completely flat here. Windy, but flat. It may sound crazy but I'd like to be able to go 75mph on a calm day if I muster up the courage. With 4.10s and an ax15 it will be spinning 2333 rpms at that speed, and 2177at 70, possibly sustained for a few hours. Is that too high for these 2.8s?


      • #4
        I'm running 3.07s with 33s and on flat land I'm fine even with a trailer. You could get away with 3.73s, but I'd still be hesitant, because at times you'll probably want just a bit more torque and the other 4 gears will have slightly better shift points imo. I'm not sure what ratios you can get, but technically probably something like a 3.9 would be ideal for your setup. You'll be ok for sure with 3.73s, but I've always heard to go just a bit bigger gear ratio if you're on the fence. I was completely sold on 3.55s and after trying an equivalent, tire to 3.73s and 33s, it's clear that 3.73s and maybe even 4.10s for my setup would be better.

        With my 3.07s and 33s in 4th gear, that's basically the same as running something a bit bigger than 4.1 and 35s 5th gear on the highway and yes it's a bit aggressive, but not crazy aggressive. 2100 RPMs is nothing to this engine.

        However running the numbers and the fact that 35s are probably closer to 34s, 3.73s wouldn't be bad. For most people, especially those that really like to off-road I'd suggest 4.10s for a slightly more versatile gear ratio. Also 35s are big and heavy wheel/tires and 70+ mph is a lot of wind resistance. However you'll move around just fine with 3.73s. I'm moving around just fine with 3.07s and 33s, not ideal, but it's WAY better than the OEM engine. 3.55s would even be a major improvement for me. So if you were 3.73s and above, you'll be just fine. However between the 2 that's a harder call for your description of your driving and per your description of what you want, I think you may prefer 3.73s over the 4.10s, but you'd hardly notice running 4.10s. If you ever want to entertain bigger tires, go 4.10s.

        Just for my curiosity, I'd love to hear what you decide.


        • #5
          Hopefully I'm overthinking this by a bunch haha. 3.92s don't seem to be available for the rear axle I'm looking at but you're right, they probably would be ideal. So between 3.73 and 4.10 I still think I'll go with the 4.10s. I do have some alterior motives in that I can run 35s with the 4.10s for a couple years with my current motor while I save enough for the 2.8. I'm just not familiar with diesels in general or their powerband, but you and your setup make it sound like it makes pretty good power anyway, atleast on the low end. I was hoping one of these guys that has built a couple would chime in with their experience. I basically just want to pay for these axles and gearing once, and of course even the carriers will be different between 3.73 and 4.10 and 3.73 won't work currently. I plan on getting the axles in the next month or so if I can get the clarity I'm after (or just make my own decision), but if you regear before then I definitely want to hear how it goes. Again, hopefully I'm overthinking it! BTW I don't think I'll ever run bigger tires than 35.


          • #6
            If you're still saving up and 4.10s is a bigger gear ratio right now, then I'd go with that, because the diesel doesn't need it, but this is a modern diesel so it's only slightly lower than you'd want if you were primarily doing highway driving. Whatever gas engine you start with will need as much gear as possible. My 3.07s even with 29" tires made 5th gear only useful on downhills. 65+ with that gearing was almost impossible and most trips were done 45-60mph. The R2.8 still doesn't love 3.07s, but I can maintain 80 with a little shifting. Jeeps aren't something I look to tryout top speeds though, but I've done 85 mph pretty easily. Playing around being geared more reasonably, I'm sure I could make the rest of the Jeep fall apart before the engine is done, but it's not high HP, so it doesn't move you like a V8 or some of the newer diesel trucks. It doesn't pin you in your seat, but if you look away it'll just keep on pulling.

            For the R2.8 the differences are going to be subtle, because you have enough low end torque to do the job. 3.73 will very likely maintain speed up steep hills in 5th gear and may be able to speed up and pass some, but if you are able it will be very slow. 4.10s will have just a little more juice to get up hills.