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

Discussion in 'Chevy C/K Truck Forum' started by 1977GMCck25, Apr 16, 2010.

  1. 1977GMCck25

    1977GMCck25 New Member

    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: Apr 16, 2010
  2. 1977GMCck25

    1977GMCck25 New Member

    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. Big75

    Big75 Rockstar 4 Years 500 Posts

    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.
     
  4. Big75

    Big75 Rockstar 4 Years 500 Posts

    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.
     
  5. Big75

    Big75 Rockstar 4 Years 500 Posts

    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.
     
  6. 1977GMCck25

    1977GMCck25 New Member

    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. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    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.
     
  8. 1977GMCck25

    1977GMCck25 New Member

    yea its 4x4 with 265/75/16 tires
     
  9. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    I'd go at least to 3.42s with those tires. Will really help your mileage.
     
  10. 1977GMCck25

    1977GMCck25 New Member

    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: Apr 19, 2010

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