REGEARING for larger tires/

Discussion in 'Lift Kit Discussion' started by tommudd, Jul 22, 2017.

  1. tommudd

    tommudd Moderator Staff Member Moderator

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    POSTED THIS ON FACEBOOK AFTER GETTING A PM FROM SOMEONE WHO SAID THEY THOUGHT THEY WOULD GET BETTER MILEAGE WITH 32'S AND STOCK GEARING SINCE THEY WOULD BE TURNING LESS RPM'S

    REGEARING FOR LARGER TIRES
    RPMS not exact but close and being used for discussion purposes only not exact RPMs
    Recently I read in one post where the owner was running 32s with stock gearing and told someone it was the best since it gave him more gas mileage.
    Now years ago with carb'd cars etc that sort of was right but not anymore . With the modern computer controlled vehicles of today manufacturers are trying to wring out the very best gas mileage they can , the power band in most new vehicles is in a very narrow RPM range.
    We'll take the KJ , stock with 29 inch tall tires and 3.73 gears say it runs at 2800 RPM, move up to a 32 inch tire and it drops that down to say 2500 RPM. Now you are lugging the engine , working the transmission harder since the computer is trying to find the best gear etc. Plus now you have heavier tires, more than likely wheels are heavier as well plus skids etc .
    NOW taking that same KJ with 32s and install 4.56s. It will run about 3100 RPM, have the power to pull the extra weight of bumpers, wheels/ tires/ skids etc and not be shifting on every little incline . 32s and 4.10s is about 2800 which is good to a point, but think of pulling the extra weight .
    I'm running 4.10s on my 03 with 265-70-16s, ( 30.6 inches tall so considered in some circles to be 31s) and its almost perfect all around, pulling trailer, on interstates and backroads. About 2900 RPM with that combo.
    My 04 when I went to 32s and still stock geared I hated it, for a couple of weeks it would downshift even on small hills on the interstate, had to run with the OD off all the time. Then regeared to 4.10s and once again fun to drive.
    So in the end, if you want to run larger tires etc plan on regearing. If you don't believe me look up the charts for yourself, many companies have them online. I've regeared every vehicle I have lifted and installed larger tires on for years for longevity, more power and mileage and overall fun factor. Rev. Mudd waiting....on the collection plate to be passed LOL Sermon over
     
  2. libordemise

    libordemise Full Access Member

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    Amen

    Sent from my LG-K210 using Tapatalk
     
  3. sota

    sota Full Access Member

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    some MPH numbers to go with the RPMS would be helpful.
     
  4. tommudd

    tommudd Moderator Staff Member Moderator

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    Sure it would but didn't have time to do a full blown Sheldon Cooper explanation
    So there ya go , you can do it:icon_razz::icon_razz::icon_razz::gr_grin::gr_grin:
     
    Last edited: Jul 22, 2017
  5. kejobe

    kejobe Full Access Member

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    While I agree that regearing is the best thing to do when going to larger tires, IMO, there are some instances where it becomes a moot point.

    Example, me. I bought the KJ as a winter vehicle since my normal DD is a '14 Maxima that I had lowered 1.5". it's not the best for winter. The KJ needed tires on it so after some research I had found that 265/70/16's should fit with the lift on it. Got them and they do, minus some rubbing on the skirting in front of the wheel and the sway bar at lock. I can live with that.

    Now to the direct moot point. It's mainly a winter vehicle and maybe a mild (to the low end) off road rig at some point. For the cost to regear for a bad weather vehicle that will only get driven 4ish months out of the year, I cannot justify the cost.

    That being said, if it was a DD year round and in better condition all around, then yes, I would regear.
     
  6. tommudd

    tommudd Moderator Staff Member Moderator

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    I've regeared ones that I drove less than 4000 miles a year
    On the 03, I have less than 375 bucks in the front and rear 4.10 diffs with less than 50,000 miles when I bought them couple of years back, so great investment even if I didn't drive much, shop around
     
  7. tjkj2002

    tjkj2002 Full Access Member

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    Now this is for a KJ with a 45RFE trans................

    1-3.73 gears,235/70R16 tires,and 45REF trans at 3000 rpm's = 92mph
    2-4.10 gears,265/75R16 tires,and 45RFE trans at 3000 rpm's = 92mph
    3-5.13 gears,315/75R16 tires,and 45RFE trans at 3000rpm's = 80mph

    #1 is stock at about 4000lbs and never got better then 21mpg's.
    #2 was lifted at 5500lbs and got 22mpg's(21.9 actual best).78mph was best cruise speed at this stage to get that mpg's,at 65mph it would get 12mpg's.
    #3 is SFA'd at 6400lbs and can get 18mpg's,this at 65mph which turned the same rpm's as 78mph with 32" tires and 4.10's.

    When I first lifted my KJ I was running 265/75R16's with the stock 3.73's,once I geared to 4.10's I went from 6-7mpg city to 12-13mpg's city and 16mpg's highway to 22mpg's highway.

    Even if driven only a few months you will recover your re-gear cost pretty fast in gas alone.
     
  8. Jim McClain

    Jim McClain Full Access Member

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    My tires are not quite 32s, I have a full set of good skids and plan to add the gas tank skid at some point. I'd like to get a front bumper and a winch and not counting the mountain trails I like to ride, there's also plenty of hilly highway. I might even be thinking about getting a small travel trailer.

    So, are you suggesting all that might be better with 4.56 gears, or will 4.10s work?

    I've looked at differential products on JBA and I'm confused by all the parts available. I know I need to get the ring and pinion gears for both front and rear, but what else is necessary? I'd also like to add a Detroit locker, but I don't think my style of driving requires a locker for both front and rear. Which diff should get the locker?
     
  9. tommudd

    tommudd Moderator Staff Member Moderator

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    If you are seriously looking at getting a trailer then by all means I would shoot for the 4.56s. I pull a 6 x 10 enclosed quite a bit and when loaded I know with 32s and 4.10s its "ok" but would be way better with even lower gears than 4.10s. If just normal driving 32s and 4.10s do ok , but there are plenty of times that its shifting down when I know that with lower gears it would of walked up over the hill without shifting down.
    The 03 with 4.10s and the 265-70-16s ( 30.6 inches tall ) just about is perfect as far as maintaining speed on the highway. Keep thinking I'll go with a larger tire on it but really liking the way it runs with that combo.
    So did that answer your question, I doubt fully since it does take a large chunk of money to go to JBAs steel diff .
    Same as some thinking its not worth it to install aftermarket springs and shocks on a KJ to lift it where you can buy a spacer lift for 200 bucks.
     
  10. Jim McClain

    Jim McClain Full Access Member

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    That answered all but this part:
    I wasn't thinking the JBA steel diff, I was thinking only the various parts required to convert my existing diffs. I'll continue to try to find a front diff at a nearby dismantler so that can be converted and I won't have so much down-time. My problem is not knowing exactly what parts I need to do the conversion and what diff should get the locker.
     
  11. HoosierJeeper

    HoosierJeeper Gold Supporter/Admin Staff Member Administrator

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    You can't do anything lower than 4:10s without a JBA diff, 4:56 won't work in the stock diff...it's a slightly different variant of the Dana 30. Only Mopar 4:10s will work. The steel one allows you to use any Dana 30 gear set, as long as you get the right spline count etc.
     
  12. tommudd

    tommudd Moderator Staff Member Moderator

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    Sorry thought you were asking of going to 4.56s
    Getting another front diff would be easier for you then have gears done and install. Shorter time being down .
    Basically just the 4.10 gears and install kit which includes everything to install the new gears
    front
    Spicer 4.10 Ring and Pinion
    $350.00
    KJ - Ring and Pinion master install kit
    $139.95

    rear
    KJ - Master install Ring and Pinion rear diff.
    $85.95
    Ring and Pinion 4.10
    $379.99
    there is that much for you Jim, I think the prices should be pretty much the same , haven't updated my " master catalog" in a couple of months ( need to do that soon )
    But those are JBA prices, may shop around
    The same with lockers depending on what you want