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  1. #1

    Default 4:10s to 3:73s in a 14 bolt rear end

    i have a 1977 gmc K25 with 4:10s or 4:56s and a th350 tranny i want to put 3:73 or 3:55s i was wondering if anyone knows if this will work without doing any modifications to the truck how much faster will i beable to go with diffrent gears
    Last edited by 1977GMCck25; 04-16-2010 at 12:03 PM.

  2. #2

    Default gears

    how do you tell what gears are in my truck, ive taken the diff cover off but i still cant tell what ones are in there, its a 1977 gmc k25 with a 14 bolt

  3. #3
    Master Mechanic Big75's Avatar
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    4.10 is my guess with the 14 bolt. make a mark on one tooth on the ring, and one on the pinion. rotate one tire to count the teeth. divide ring teeth by pinion teeth.
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    Master Mechanic Big75's Avatar
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    you mean like top speed faster? not much. probably not enough mph more to make it worth it. you know you get better acceleration with the 4s than with the 3s, right? going from 4.56 to 3.73 will feel like you lost half your horsepower.
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  5. #5
    Master Mechanic Big75's Avatar
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    and I dont know about the 14 bolt but if it's set up like the 12 bolt you may need to get a new carrier to make such a drastic ratio jump. for me to change my 3.07 to a 4.10 the ring gear is thicker which means a new carrier.

    when in doubt, google. pirate4x4.com would have some good write ups on this I bet.
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  6. #6

    Default

    yea i mean top speed, im going to be driving this for a high school truck and i want to be able to go over 55 a a reasoble rpm

  7. #7

    Default

    First of all, find out what your current gears are. Pull the diff cover. Count the ring teeth and divide by the number of pinion teeth. Or it may be marked on the side of the ring gear.

    Second, what size tires are you running? Four wheel drive? You will have to change both gear sets.

    If you currently have 4.10s (most likely), changing to 3.73s will increase your speed by about 10%. So whatever RPM you were doing 50 at, you will now do 55. Changing from 4.56 to 3.73 or 4.10 to 3.42 will give a much more dramatic drop in RPM/increase in top end.
    When you hear hoofbeats, look for horses not zebras.

  8. #8

    Default

    yea its 4x4 with 265/75/16 tires

  9. #9

    Default

    Quote Originally Posted by 1977GMCck25 View Post
    yea its 4x4 with 265/75/16 tires
    I'd go at least to 3.42s with those tires. Will really help your mileage.
    When you hear hoofbeats, look for horses not zebras.

  10. #10

    Default

    what do you recomend 3.73 or 3.42, because i still want to beable to do some off roading and have some low to mid end power with alright milage
    Last edited by 1977GMCck25; 04-19-2010 at 12:11 PM.

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