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How hard is it to swap my 3.08 gears to 3.73 gears?

Discussion in 'Differential & Driveline' started by Ewhitaker0020, Jun 14, 2020.

  1. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    I've been told several times that my 84 K10 with a weak 305 would benefit from swapping my puny 3.08 gears over to 3.73 gears. I've been told my a bunch of people that it will make a huge difference to the way my truck feels on the road. I have a 305 with a freshly rebuilt 700r4 transmission and would love for it to not feel like a tired, old, sluggish truck.

    What all would I have to change? Surely I would have to do something up front to or my 4wd would be all out of whack.

    Opinions and personal experiences are very welcome as I'm not really sure what to expect. Since I have overdrive will I notice a huge dip in mpg on the highway?
     
  2. Ricko1966

    Ricko1966 Full Access Member

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    if you change the rear ring and pinion you need to change the front to a matching ratio.Now just for fun, swapping tire sizes is like swapping gears taller tires are more like highway gears shorter tires give more low end.There is a formula, which I don't have memorized.As to how tire size affects final drive ratio. Look online for the formula and see if you can borrow from a friend, buy cheap from a junkyard, whatever some shorter tires to see if it's enough change for you.Im pretty sure. 14 inch mazda b2200 wheels and tires will bolt on.It will look stupid but you can see what the ratio change feels like.
     
    Last edited: Jun 14, 2020
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  3. Curt

    Curt Junior Member

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    Pretty good idea to change the tires to get as close to desired gear ratio,if possible.I would ask myself what the ultimate use of the truck will be.

    Considering the the cost of both ring and pinion changes,rebuild kits and especially labor,if not done yourself,you could very well be at the cost of a mild 350 build.
     
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  4. Rusty Nail

    Rusty Nail Whatever!

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    Yep.

    First I would choose what size tire you will use on the truck.

    3.73 over 3.08 will feel like a 50 hp kick in the nuts but may not be the right choice.
    Often not enough.
    If you choose 33 inch tires for example, you'd be better off at 4.56 or 4.10 in most applications.

    Again, pick your permanent tire size first and start saving money because gear changes are expensive but worth every dime.

    Probably wont have to change anything else but you have to do both differentials.

    https://www.morris4x4center.com/jee...n=cj&cjevent=4c991a88bc1411e98058020f0a1c0e0f


    Hope this helps!
     
    Last edited: Jun 14, 2020
  5. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    So, it looks like of I do 31" tires and 4.11 gears then I would be sitting right in a green square at 2896 rpm. Do I want to be in the green to get a boost in how the truck feels? I don't even know where my truck would be at right now on the chart. I have 3.08 gears and 30" tires. Just roughly guessing makes me think I would be 4 squares above a yellow which would explain the huge lack of power.

    Let's just say I went with 31" tires and 4.11 gears and landed on a green square. What would that do to my driving at highway speeds? Could I still drive the truck at 75 mph on the interstate? I have an auto tranny with overdrive. I need more power, but I don't want it to be screaming down the road.
     
  6. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    Ultimately the truck is for cruising and driving to work occasionally just for fun (1-2 hour drive each way)

    But I want to be able to tow a trailer with it as well occasionally. I tow a 16 foot trailer with a 700 pound mower and the truck will take forever to build speed. I can floor the pedal and barely gain speed the way it sits. I think my wife's 4 cylinder kia Sorento would do just as good if not better.

    I do not want to go huge on tires. I don't want to lift the truck and I can't remember if 33's will fit without rubbing or not. If they'll fit fine without rubbing then I would want to go with them. If not then I would probably go with 31's.

    I used a chart somebody provided and to get in the green power band I would need 31's and 4.11 gears. If I went with 33's then I would need 4.27 gears, which I'm not even sure is a thing on Chevy trucks of that age.
     
  7. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    Right now I have 30's on my truck. Using a chart somebody else posted it looks like 30's and 3.08 gears put me way above the highway cruising band range. So I'm way into the low power max mpg section of the chart. No wonder it feels like I have no power. Looks like if I went with 31" tires then I should go with 4.11 gears. If I went with 33" tires then 4.27 gears of that's an option with Chevy.
     
  8. 75gmck25

    75gmck25 Full Access Member

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    I have 4.10 gears and 31” tires on my K25 with TH350. It’s “busy” at 70 and about 3100 rpm, but not that bad. Your overdrive 700R4 would drop rpm to .70 of 3rd gear rpm, so it should be about 2200 rpm. This sounds fine to me.

    4.10 gears are common in 3/4 ton squarebodies, but much harder to find in a 1/2 ton. Many 1 tons have 4.56 gears. Quite a few 1/2 tons had 3.73 in certain years. If you swapped both axles from a 3/4 or one ton to get the gears, it’s a straightforward swap, but you get 8 bolt wheel hubs. That means you have to also get the wheels as part of the swap, or buy new ones.

    Bruce
     
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  9. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    I actually kinda want to swap 3/4 ton axles into my truck anyways. If I found a 3/4 ton truck is there any way for me to know what gearing it has?

    I wouldn't mind swapping my wheels. It'll be about time for me to get new tires then anyways, so the wheels won't be too much more.
     
  10. RecklessWOT

    RecklessWOT Full Access Member

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    This is basically exactly what my response was going to be. Couldn't have said it better myself.
     
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  11. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    Can I out 33's under a stock 84 k10? I don't want any issues with rubbing or clearance. If there will be issues I'll go with 31's
     
  12. RecklessWOT

    RecklessWOT Full Access Member

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    You can get 4.27s, but afaik they were not offered factory so you won't find axles with that ratio in a junkyard, you'll have to buy the gearsets and have them swapped out
     
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  13. RecklessWOT

    RecklessWOT Full Access Member

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    31s fit no problem, some guys even have them on 2wd trucks. You might be able to squeeze some 33s in there but there is a good chance they will rub up front when turning and hitting bumps. 31s were actually the factory tire size on a lot of trucks. I have a 2" lift on my suburban and have miles of room with my 33s. So it's close with a stock setup
     
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  14. Ewhitaker0020

    Ewhitaker0020 Full Access Member

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    Sounds like I'll be going with the 31's then. Are all I need for an axle swap are axles? Do you know what years I should look for? I would also be okay taking it to a speciality gear shop (if they exist) and have them swap out just the gears for me so I can get it done cheaply.
     
  15. Ricko1966

    Ricko1966 Full Access Member

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    Instead of saying just for fun here, I should have said just for diagnostic purposes.If you can borrow some lowered mini truck wheels and tires this will give you the feel of the gear change.Your trucks gonna look stupid with 195-60-14s on it but if it is still is a slug then you need to evaluate.Why does it feel like a tired sluggish truck? I just used that tire size as n example, but maybe truck is tired and some engine work needs to happen. I fully agree 3.73 is a 50hp kick in the nuts but if the engines tired.Thats where I'd start. Junkyard wheels and tires are stupid cheap around here and you only need 2 if you run in 2 high to feel the difference. A gear change feels awesome on a healthy motor.
    In 2 months I don't want to see the post.I put 4.56 gears in my truck and it's still doggy.Whats wrong with it?
     
    Last edited: Jun 14, 2020
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