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Thread: Has anyone done this gear swap?

  1. #1
    Join Date
    Sep 2008
    Location
    Stevens Pointski, Wisconsin
    Posts
    1,350

    Default Has anyone done this gear swap?

    Has anyone dropped that much coin and just swapped out the stock ring gears and pinions to the 4.56?

    http://www.bjsoffroad.com/prod-957.htm

    Just curious how much more MPG or top end you can gain in otherwise stock truck.

    Scott
    '67 M715 '67 M725 '69 M726 (x2)

    "it's cheap and you get all you can shove in your pie-hole" --Kozmo 12-10-13

  2. #2
    Join Date
    Sep 2008
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    Stevens Pointski, Wisconsin
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    Default

    Finally I found what I was looking for, thanks Randyscycle:

    With my truck I have the stock axles, regeared to 4.56, stock drum brakes, MRW wheels and Michelin XL's and the stock NP200 transfer case with the right output still going to the rear.

    I can now drive 60 MPH all day, and with a Jeep ZJ master cylinder/booster it stops as well as any other vehicle I currently own, unless I get the brakes really hot, then they fade as any drum brakes will tend to do. I've never felt unsafe though. The NP200 with the 4.56's never heats up like it used to, although it is still noisy because of the stock output location. I have a temp gauge in it so I can actually watch the oil temp to see what its doing. Since you already have the Selectros and a rear locker, I'd go the regear route, personally. For mild wheeling, you will probably never break the axles. The front is approximately Dana 44 in strength where the axle shafts are concerned. The gears and carrier are still D60 stuff. The 70 in the rear is pretty much bulletproof. Of course anything can be broken
    .


    You didn't mention whether is was the 230 yet, tho.

    Scott


    __________________
    '67 M715 '67 M725 '69 M726 (x2)

    "it's cheap and you get all you can shove in your pie-hole" --Kozmo 12-10-13

  3. #3
    Join Date
    Sep 2010
    Location
    Middleville, mi.
    Posts
    1,245

    Default

    Quote Originally Posted by fishtail View Post
    Has anyone dropped that much coin and just swapped out the stock ring gears and pinions to the 4.56?

    http://www.bjsoffroad.com/prod-957.htm

    Just curious how much more MPG or top end you can gain in otherwise stock truck.

    Scott
    I haven't done it but the rpm decrease is significant at a given speed.

    I made a spread sheet with the formula in it and ran it. I went from 46 MPH to 59 with that gear change when I checked it.

    The formula is speed=rpm/(tire diameter x final gear ratio x axle ratio)

    In my example it was 2500/(36x1x4.56) compared to 2500/(36x1x5.87)

    I would think the only reason for the swap would be to gain the ability to have the higher speed. I don't think there would be a payback of fuel economy soon enough to justify the expense.

  4. #4
    Join Date
    Sep 2010
    Location
    Middleville, mi.
    Posts
    1,245

    Default

    Oops my formula was actually written wrong It should have been rpm x tire diameter / (final gear ratio x axle ratio x 323.13)

  5. #5
    Join Date
    Dec 2003
    Location
    Giddings, Texas
    Posts
    7,729

    Default

    Randy's Ring and Pinion has a great plug in formula on their site:http://www.ringpinion.com/Calculators/Calculators.aspx

    They even have an app for smart phones. I use it all the time in my different vehicles to know how fast I am really going.
    Remember if you didn't build it you can't call it yours.

    6.2 powered M715, 5 M1009's, M416, 2 M101's, 2 M105's, 3 M35's, M1007 6.5 turbo Suburban project called Cowdog.

    https://www.youtube.com/channel/UCCz...HGkBCfhXZ5iuaw

  6. #6
    Join Date
    Jul 2007
    Location
    Rhoadesville, Virginia (five miles from no place)
    Posts
    5,125

    Default

    Quote Originally Posted by fishtail View Post
    Finally I found what I was looking for, thanks Randyscycle:

    With my truck I have the stock axles, regeared to 4.56, stock drum brakes, MRW wheels and Michelin XL's and the stock NP200 transfer case with the right output still going to the rear.

    I can now drive 60 MPH all day, and with a Jeep ZJ master cylinder/booster it stops as well as any other vehicle I currently own, unless I get the brakes really hot, then they fade as any drum brakes will tend to do. I've never felt unsafe though. The NP200 with the 4.56's never heats up like it used to, although it is still noisy because of the stock output location. I have a temp gauge in it so I can actually watch the oil temp to see what its doing. Since you already have the Selectros and a rear locker, I'd go the regear route, personally. For mild wheeling, you will probably never break the axles. The front is approximately Dana 44 in strength where the axle shafts are concerned. The gears and carrier are still D60 stuff. The 70 in the rear is pretty much bulletproof. Of course anything can be broken
    .


    You didn't mention whether is was the 230 yet, tho.

    Scott


    __________________

    I'm running a Big Block Chevy and a TH400 transmission. Stock driveline from there out excepting the 4.56 gears. Like I said in that post, for the way I use the truck, those axles will probably last as long as I have it and they will most likely stay as I have a few spare complete front and rear axles now too.

    I have another truck that at some point I plan to build and it will most likely go the 6BT route, so that one will get an axle swap too, to something modern for the disc brakes, mainly.
    "Free advice is worth what you pay for it."™

  7. #7
    Join Date
    Jun 2009
    Location
    Litchfield Illinois
    Posts
    596

    Default

    I really like this site for gear calculations. You can do side by side comparisons. It calculates about any way you can imagine.

    http://www.grimmjeeper.com/gears.html

  8. #8

    Default

    4.56 gears for almost $1000 is crazy plus you'll have to do the labor. If it was me I and I really wanted 4.56 gears I'd just get a set of CUCV axles which already are set up for this ratio, have a rear locker, are stronger, have disc brakes up front, and have the common 8x6.5 bolt pattern. This swap has been done many times on this site as well as even in JP magazine. Think you find find sets for $1500 ($1100 front, $400 rear) As for MPG you'd have to put a lot of miles to make up the cost difference.

    Easiest way to change your final drive ratio is to change your tire size but it sounds like your trying to keep things stock.
    TBI 454 and chunked up XL's.

  9. #9
    Join Date
    Jul 2007
    Location
    Rhoadesville, Virginia (five miles from no place)
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    5,125

    Default

    Quote Originally Posted by HUMMER103 View Post
    4.56 gears for almost $1000 is crazy plus you'll have to do the labor. If it was me I and I really wanted 4.56 gears I'd just get a set of CUCV axles which already are set up for this ratio, have a rear locker, are stronger, have disc brakes up front, and have the common 8x6.5 bolt pattern. This swap has been done many times on this site as well as even in JP magazine. Think you find find sets for $1500 ($1100 front, $400 rear) As for MPG you'd have to put a lot of miles to make up the cost difference.

    Easiest way to change your final drive ratio is to change your tire size but it sounds like your trying to keep things stock.
    And you'll also need to do the labor with an axle swap as well, which is a lot of work too. Those CUCV axle sets for cheap are starting to dry up already.

    Even changing up to 38" tires only dropped the RPM of my Chevy 396 by 500 RPM or so and gave it a top speed of around 55 MPH with a still overheating NP200.

    Not saying stock axles re-geared is the best way, mind you, and if you really sit down and do the math, none of the options are a real money saver, with MPG less than 10 with a gas engine, even re-geared. It all depends on what you want to do. With a pile of stock axles out back, plenty of spare stock brake parts, and a set of Marsh wheels, for me it was an easy choice and it works quite well.
    "Free advice is worth what you pay for it."™

  10. #10

    Default I did the swap

    it was expensive everything came from Randy's and a local shop with a good rep installed. I have a stock motor and NP205 transfer case. Road speed vs RPM is significant but without a tach I cant say exactly. I am sure there is a lot less stress on the engine at 55 then before. Between flat lands and the Blue ridge its about 30 miles so its a much more enjoyable ride.

    Tony

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