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Gears and other stuff

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Cornfed-90K5, Apr 28, 2000.

  1. Cornfed-90K5

    Cornfed-90K5 Registered Member

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    If anyone out there is considering a gear swap and has 33" or 35" tires I suggest going with 4.56's. It gave me all around great performance much better crawl ratio, with my torque converter added into the picture my crawl ratio is about 75:1. I also noticed a vast increase in top end acceleration power. I even got an unexpected bonus, my mileage increased, I also recalibrated my speedo.

    I would also like to thank everyone who gave me suggestions about changing my ball-joints, boy what a difference!!! I would also like to tip my hat to Colorado K5 for giving full-size Blazer enthusiasts like myself a place to talk and help each other. Thanks again.



    Cornfed-90K5
    1990 K5 Blazer
    1977 Jeep CJ5
     
  2. mudfanatic

    mudfanatic 1/2 ton status

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    Are you running 35'S? I am running 33's and would like 4.10 or 4.11'S I think that for 33'S this would be the best all around gearing... Am I wrong? I sure wouldn't want to spend that kind of money just to find out that I'd have been happier with 4.56'S
    '77 K5

    <font color=red>mudfanatic</font color=red>[​IMG]
     
  3. 73/75 k5

    73/75 k5 Registered Member

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    I've got 4.11s in my 75 and I run 35in MTs and 33in ATs for street. Your rpm on the freeway will run about 3000rpm at about 70-75mph. I did not notice much difference in rpm drop between the two. I would like to get an overdrive tranny but lack the fundings. My 73 has 4.56s. Great for wheeling but tough on the freeway. Running 35in MTs on the 73 but plan on 38s. If you don't mind driving in the slow lane on your way to your favorite trail run 4.56 or even 4.88. Take it easy.
     
  4. Cornfed-90K5

    Cornfed-90K5 Registered Member

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    At the moment I have 33's but will be putting 35"s on this summer, there is only about a 180 rpm difference between the 2 tire sizes at 60 mph. I think if you are going to spend the money you will be happier with the 4.56's.


    The all around perfomance diff is well worth it, I had 3.73's before.

    My tranny also has a 30% overdrive, my rpm at 70 mph is about 2350-2400 rpm
    Cornfed-90K5
    1990 K5 Blazer
    1977 Jeep CJ5
     
  5. laketex

    laketex 3/4 ton status

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    How did you get 75:1 for your crawl? Maybe I've been trying to multiply it all together wrong. I'm thinking the way you do it is this?

    axle gear ratio x transmission 1st gear x T-case low range x 2nd T-case where applicable

    Therefore mine is 4.11x2.52x2.0 = 20.7:1

    Even if I go with the ORD doubler somewhere down the road, it will be only around 41.4:1

    I can just hope for the doubler coupled with a set of custom 4:1 NP205 gears yet to be designed, to set me around 83:1

    But am I wrong here? If I'm not, how the heck did you get a 75:1 ratio?????
    by my math I dunno your setup, but with 4.56 gears, tranny 1st at 3.5, and 208 case to be 43:1.

    Hmmm...someone straighten me out here [​IMG]

    [​IMG][​IMG]
    Durant, Ok
    '79 Blazer in progress
     
  6. 6.2Blazer

    6.2Blazer 1/2 ton status

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    The torque converter will give you an additional gear reduction of around 2:1.
     
  7. Cornfed-90K5

    Cornfed-90K5 Registered Member

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    You are correct first gear x t-case x axle gear, but with an automatic you can multiply this number by at least 2 and I have been told that with the non-lock up tcc on my 700R4 I can even multiply by up to 2.5, with an automatic, however, the number goes down as the rpm's climb. The 75:1 is multiplying by 2 to be on the more modest side. If I go 2.5 it is around 96:1 my friends are all jealous because my jeep and my blazer out crawl anything they have. My Jeep is 123:1. It has an NP435 6.73:1, Dana 300 (4:1), and 4.56's.
    My Blazer is first gear (3.06) x low t-case (2.72) x axle ratio (4.56) x tcc (2.5)=96:1

    Cornfed-90K5
    1990 K5 Blazer
    1977 Jeep CJ5
     
  8. blazer72

    blazer72 1/2 ton status

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    I have have the fun stuff from ORD NV4500(6.37)X 203(2.1)X 205(1.98)X the 3.73 in the diff's = 98.9 to 1 the rear drive shafts 23.5 inches long! bell houseing to the rearout put is 54 inches. I have the standerd gearing for hyway yet I can gear down for off road the realy fun thing now is making the Rearend in back live. ARBs will not takr the tork in back had to go to a detroit in the back.
     
  9. titanic

    titanic 1/2 ton status

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    I have a doubler with a 465 tranny,203 gears, then 205 gears ,5.13 diff. gears.....132:1 crawl It helps when you have 44's
     
  10. blazer72

    blazer72 1/2 ton status

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    I run 35s and drive it from Kansas City to Moab or to the K5 meet this summer
     
  11. laketex

    laketex 3/4 ton status

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    Ok so I'm understanding now...so with my TH350 and non lockup, I can expect between 2.0 and 2.5 reduction? That's great! I'm a lot more stoked now. Thanks for the info guys.

    [​IMG][​IMG]
    Durant, Ok
    '79 Blazer in progress
     
  12. 6.2Blazer

    6.2Blazer 1/2 ton status

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    I don't think the lock-up vs. non lock-up torque converters will have any effect in the initial gearing (when first taking off). The only difference is that the lock-up converters will, obviously, lock-up once a steady cruising speed is obtained so there is no slippage (a.k.a. gear reduction) and therefore better fuel economy.
     

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