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    How to: What Gear Ratio is My Rear-end? (The Basics)


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    How to: What Gear Ratio is My Rear-end? (The Basics) Empty How to: What Gear Ratio is My Rear-end? (The Basics)

    Post by DanTheVanMan Wed Aug 17, 2011 10:42 am

    What Gear Ratio is My Rear-end?

    Since I have been doing a lot of rear-end work on the 66 Panel I have learned a few quick ways to check out their ratios and thought I would share.

    You can find your gear ratio easily with this method. All you need is a jack, some jack-stands, and knowing how to count helps too.

    You're going to have to put the car in neutral or out of gear, so safety first! Do this on level ground, and be sure and chock the front wheels so the car can't move. It helps to have someone there in the car just in case...

    First, do you have a posi or limited slip rear? If you're not sure, just jack up the rear so both tires are off the ground and put your jack-stands under the axle. Now turn one of the tires. If the other tire turns the same way or doesn't turn at all, you should have a posi or limited slip. If the tire turns opposite of the way the first tire spins, then it's probably an open rear end.

    If you have a car with a posi or limited slip rear, jack up the car and get both rear tires off the ground. Now put your jack-stands under the axle. Make a line on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.

    Now count the number of turns of the drive-shaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation. (Transmission needs to be in neutral or out of gear.)

    If your car has an open rear, jack up only one rear tire and leave the other on the ground. Now put a jack-stand under the lifted side. Turn the tire and again count the number of turns of the drive-shaft, but this time you must turn the tire two full rotations.


    Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.

    - Count the turns on the drive shaft.

    Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.

    - Count the turns on the drive shaft.

    If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the drive-shaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.

    This method is very handy when looking through junkyards or swap meets. It's good to be able to figure what gear ratio you have without tearing anything apart.

    This is a quick and easy way to tell what gear ratio is in your rearend. You might not be able to tell exactly what gear you have, but you'll be very close.

    If you want to be accurate, then count the teeth.

    If you have the rearend opened up where you can see the gears, count the teeth on the ring and pinion. Divide the ring teeth number by the pinion teeth number.

    Example: 41 teeth on the ring and 10 teeth on the pinion = 4.10 gear ratio...

    Just some helpful info for those looking to replace their rear-end or just wondering what you already have without searching for the numbers and doing the math....


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    Post by AzDon Sat Jun 20, 2015 5:08 pm

    Cool tutorial Dan!
    I'd like to add a couple of other methods, if okay (?)
    I actually use your same method while winch loading the dead vehicle onto my car trailer.... Many times, there is not a clear view of the driveshaft, so I position a long ziptie around the shaft with the tail pointing straight down so that the progress can be noted from beside the moving car (I also do this same deal with a driver in a running car)
    Another thing I saw somewhere is that ring gears have the number of teeth for both gears of the set stamped into the outside perimeter surface from which you can figure the ratio.... not sure if this is universal, but easy enough to look when the cover is off and easier than counting pinion teeth......
    Also, my van has a SPID tag with the options, including rear ratio, on the inside wall near where the spare goes.....

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