How to Calculate Gear Ratio wo Removing Covers

Discussion in 'Differential & Driveline' started by HotRodPC, Aug 6, 2012.

  1. HotRodPC

    HotRodPC Administrator Staff Member

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    With some simple math, anyone can determine your gear ratio without removing axle covers. You will need to know a bit about fraction and decimal conversion, but I'll put up some info here to make that easy for you. Most of are aware of the basics without even thinking about it. Examples, being 1/4 is .25, 1/2 is .50 and 3/4 is .75.

    On to the good stuff. First thing you want to do is figure out if you have a locker, spool, some Limited Slip Diffs (LSD) or locked axles as in Lincoln Locked or welded spider gears making the open diff a locked axle OR, if you have an open differential or Limited Slip Diff or Locking Diff that doesn't lock the axles until a predetermined set of events happens to make them lock up.

    This is easy to do. FIRST chock the front wheels in both front of the tire and rear of tires so it doesn't roll since your E brake goes to the rear wheels. Jack up the rear end with both wheels off the ground. I usually use a floor jack in the center of the axle. With both rear wheels off the ground. Spin one wheel. If the other wheel spins in opposite directions, then you likely have an Open differential without a locker or LSD, but G80 Locking Diff does also spin in opposite directions, so you could still have locking differential. That doesn't matter for figuring gear ratio though. Bottom line here, you're trying to determine if the wheels spin in opposite directions, or Not. If you go to spin the one wheel, and it doesn't turn at all, then you'll need to put the transmission in Neutral, and this is likely going to mean you have a locker, good LSD unit or welded spider gears. After the trans in N, spin the one wheel, and it's likely to have both rear wheels spinning in the same direction.

    It's important to know this, if the wheels spin in opposite direction or if they spin the same direction. If they spin the same direction, then it's even easier to determine the gear ratio. If they spin in opposite directions, then you have another step of multiplication to do. So lets do the easy one first, and assume the wheels spin in the same direction.


    I use the valve stem on the tire as marker and start my wheel spin count at 12, 3, 6 or 9 o'clock position. Whichever position I have the pinion yoke or U joint either straight across or straight up and down. It just makes it easier for an accurate count. Then you're off. Make 1 full tire spin by hand and count how many times the driveshaft, or the pinion yoke turns. If the driveshaft spins:


    Wheels Spin in the SAME direction.

    A hair over 2 1/2 times or 2.5 times, then safe to say ratio is 2.56:1
    A hair under 2 3/4 times or 2.75 times, then safe to say ratio is 2.73:1
    A bit over 3 times, then safe to say ratio is 3.08 or 3.07:1
    A hair over 3 1/2 or 3.5 times, = 3.54 or 3.55:1
    A hair under 3 1/2 or 3.5 times, = 3.42:1
    A hair under 3 3/4 or 3.75 times = 3.73:1
    A bit over 4 times = 4.10 or 4.11:1
    A hair over 4 1/2 or 4.5 times = 4.56 or 4.57:1
    A bit under 5 times = 4.88:1
    A bit over 5 times = 5.13:1

    You should have this by now, so that's as far as I'm going and anything else is very uncommon anyway in light duty trucks and cars for sure.

    Now, if the wheels spin in opposite direction the procedure is a bit different.
    You must put 1 wheel back on the ground so it stays stationary and you can get an accurate count. This is because if the wheel is moving, that means the spider gear to the that other axle shaft is moving and you don't know exactly how much and it makes your count inaccurate. So with just one wheel in the air, transmission in Neutral so that the driveshaft is able to spin, then count the pinion yoke spins to 1 full tire revolution. Now here's the difference and the added step. Take the # of spins and multiply it by 2 and that is your gear ratio. The reason you have to multiply it by 2, is because the distance you are moving the wheel, the spider gears are spinning backwards on the other stationary axle shaft gear, cutting your pinion yoke turns in half. So if you pinion yoke spins:

    Wheels Spin in the OPPOSITE direction.

    A hair over 1 1/4 times or 1.25 times, x2 then safe to say ratio is 2.56:1
    A bit over 1 1/3 times or 1.33 times, x2 then safe to say ratio is 2.73:1
    A bit over 1 1/2 or 1.5 times, x2 the safe to say ratio is 3.08 or 3.07:1
    A hair over 1 3/4 or 1.75 times, x2 = 3.54 or 3.55:1
    A bit under 1 3/4 or 1.75 times, x2 = 3.42:1
    A heavy bit under 2 times x2 times = 3.73:1
    A bit over 2 times x2= 4.10 or 4.11:1
    A hair over 2 1/4 or 2.25 times x2= 4.56 or 4.57:1
    A bit under 2 1/2 or 2.5 times x2 = 4.88:1
    A bit over 2 1/2 or 2.5 times x2 = 5.13:1

    And that's how it's done. Now noone should need to remove their diff cover, unless you're just wanting to do a differential oil change, to calculate your gear ratio. Which, if you don't know the history of your truck, it's not a bad idea to change the gear oil, and identify which carrier you might have when the cover is off. Keep in mind, some units require special oils like synthetic, or synthetic blend, some require GL4, some GL5, and some require LSD additive, so make sure you get a good ID on your carrier type if it appears to be a Locker or LSD.
     
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  2. thaifighter

    thaifighter Banned

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    Great info!! very handy to know.... couple of questions
    #1 with an open carrier cant both wheels spin in the same direction as well
    #2 not off topic can you can you tell me what a 14bolt ff with open carrier and 4:10s needs for gear oil ........thxs in advance
    oh and any progress on your bast cabinet lol
     
    Last edited: Aug 10, 2012
  3. HotRodPC

    HotRodPC Administrator Staff Member

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    Most open diffs, but not all, but most 14b Full Float being one of the most, should be 75-90W GL4 gear oil, or there abouts. I hear gear oils are changing their multi viscosity ratings now to funky #'s but if it's rated at GL4 or better, like GL5 for example, and in the neighborhood of 90W, then it should be fine for a 14b Full Float and 14b Semi Float open differentials. Should also be the same for all the open differenetials in squarebodies. 10 bolts, 12 bolts and open fronts Dana 44 and Dana 60 too. It's when you have Lockers and Limted Slips is when you need to pay close attention to what gear oil to use. That will also vary depending on which Locker or LSD unit you have, rather than which axle it is, so proper identification of which system you have is important. Some LSD systems don't require special oil, but they require a special additive, but do pay attention to these things. They don't only matter for proper function, they matter for wear, noise and avoiding known problems. These engineers don't figure this stuff out just for the hell of it. They've done it for a reason, so use the right stuff for your application.
     
  4. thaifighter

    thaifighter Banned

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    the reason I ask question # 1 is I just bought a 14bff and upon visual inspection it appears an open carrier however when I spin the pinion they both turn in the same direction and when i spin one wheel the other doesnt spin Im aways away from finalizing instalation so I havent driven on it yet.
     
  5. HotRodPC

    HotRodPC Administrator Staff Member

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    That sound as if it has a LSD unit in it. Take a pic of it and post it up. Get us pics in say 3 different views of the carrier in different positions and see if someone can identify it. I was going to say, if it looks open but both wheels spin same direction, then it's probably Lincoln locked. But that can't be the case if spinning the pinion moves only 1 wheel.
     
  6. thaifighter

    thaifighter Banned

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    diff 1.jpg

    diff 2.jpg

    diff3.jpg when i spin the pinion both wheels do turn same direction but if i spin one wheel the other doesnt turn any ideas?:help:
     
  7. crazy4offroad

    crazy4offroad Full Access Member

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  8. HotRodPC

    HotRodPC Administrator Staff Member

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    Eaton Limited Slip in 14b FF.

    [​IMG]
     
  9. thaifighter

    thaifighter Banned

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    Last edited: Aug 11, 2012
  10. thaifighter

    thaifighter Banned

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    [URL="http://http://justchevytrucks.com.html[/URL]
     
    Last edited: Aug 11, 2012
  11. thaifighter

    thaifighter Banned

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    hope I dont find out its hooped when I go to drive on it:bawl:

    It doesnt look like its as easy to explode as the 9.5 gov lock its allot more stout looking and the little govener spring clip is inside the 2 halves

    yeah I dont know whats up with that link but if you go to justchevytrucks.com and then go to" parts pictures" then" differentials" theres a pretty good pic of what looks like mine


    Oh and thanks again for the help! :worship:
     
    Last edited: Aug 11, 2012
  12. chevydude

    chevydude Full Access Member

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    This is some great information thanks.
     
  13. roadwolf

    roadwolf Full Access Member

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    i tried to check this on my front axle, but i am on the case #3 wheel of the opposite side don't speen perhap's it's because of the auto-lock hub on thre front axle.

    speening 1 revolution on the right front wheel makes the front yoke revs 1 1/2 times.

    is this this formula that works with may case ?

    A bit over 1 1/2 or 1.5 times, x2 the safe to say ratio is 3.08 or 3.07:1

    i wanted to check this before putting back a front drive shaft as i don't know why it has been removed.

    it is the same for the rear the opposite wheel don't seems but i have the same count.
     
    Last edited: Jun 22, 2013
  14. HotRodPC

    HotRodPC Administrator Staff Member

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    Sounds correct. Would be a 3.0X ratio.
     
  15. roadwolf

    roadwolf Full Access Member

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    ok thanks,

    i just put the front drive shaft, and the 4x4 seems to work, i use it the "jump" from the jack stand on the rear axle.

    3.07 + 35 tires + a poor 305 now i know why it is not a quater mile winner
     

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