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Gear Size?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by JP's K5, Jan 26, 2003.

  1. JP's K5

    JP's K5 Registered Member

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    Getting ready to do a gear swap and could use some suggestions. I have an '89 K5 with 3/4 ton runnig gear and a 4.11 ratio. I have a 700R4 and I'm running 35's. I need to change to a lower gear. In overdrive I'm only running at about 1800 rpm at 65 mph. My tranny constantly locks and unlocks which causes a ton of heat. I'm undecide if I should go with a 4.88 or a 5.13 ratio. I'm not on the highway a whole lot but would still like a little bit of gas mileage. I tow some and could use the power. Any suggestions on a good ratio? I hear the 700R4 has quite an overdrive, would 5.13s be too much?
    Thanks,
    Jared /forums/images/graemlins/1zhelp.gif
     
  2. mudhog

    mudhog THEGAME Staff Member Super Moderator

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    go with the 4:88's unless you plan on bigger than 38's if you plan on going bigger than 38's just do it right the first time and go with the 5:13's /forums/images/graemlins/smile.gif
     
  3. bablazer73

    bablazer73 1/2 ton status

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    I 2nd what mudhog said
     
  4. BigRed89

    BigRed89 1/2 ton status

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    The 700R4 has a .70 fourth gear. With that fourth gear, the 35" tires, and 65mph, your engine rpm would be as follows:
    4.56 diff - 1991rpm
    4.88 diff - 2131rpm
    5.13 diff - 2240rpm
    I would suggest going with the 4.88s.
     
  5. Lonnie

    Lonnie 1/2 ton status

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    A few things to keep in mind.....

    First if you tow reasonably heavy loads.... like a car trailer or big travel trailer, it is not recommended (check your owners manual) that you tow in O.D. as trans damage can result. Internal lube circuits can be modified to allow this. Keep this in mind with your gear ratio choice.

    Second if you are only concerned with the converter lock/unlock cycling, then you can get a lock-up kit (internal or external to trans) to keep the converter locked on the highway in higher gears. Mine is currently modified (internally) to lock at 28mph in 2nd gear (4.56's & 35's) at part throttle & stays locked in 4th gear until you are getting into the 4bbl (about 3/4 throttle, when you are under significant load). The external kits generally provide electronic lock-up anytime the trans is in O.D.

    What I've seen on my truck is that the 4.56's are nearly perfect on the street unless you are in hilly areas, where it will still downshift from O.D. on the highway on big hills. Only downfall with steeper gears is that 1st gear is already pretty low & will not be too usable unless offroad. Less O.D. would be nearly perfect (the 700 is a great trans for light vehicles, but heavy trucks are better suited to less O.D. like the 4L80E).

    Hope this helps.
     
  6. JP's K5

    JP's K5 Registered Member

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    Thanks for the help! You have all made my decision for me. I was guessing 5.13s were a bit much.
    Thanks Again
    Jared /forums/images/graemlins/thumb.gif
     

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