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Old 12-22-2002, 09:49 AM
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Post How do you tell rearend Ratio

I know there Is some way that you can turn the wheels and count the Revolutions of the Drive shaft

How do you tell the Ratio If you don't have the Rearend out of Truck,
turn wheel 2 turns on Driveshaft = 1-1/4 turns on the Tire

[ December 22, 2002: Message edited by: Bluegrass ]</p>

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Old 12-22-2002, 10:04 AM
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I don't personally trust the accuracy of the spin the wheel method. I still think the best way to know for sure 100% is to remove the cover and count the teeth on the ring and the pinion and divide the two. 37 teeth on ring/10 teeth pinion= 3.70 gearset. On a rear like the Ford 8 or 9 inch it's a little more work, you have to pull the axles and drop the third member to count the teeth.
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Old 12-22-2002, 02:23 PM
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The tire spin method is pretty accurate, I mean you can tell the difference between say 3.08 and a 3.36 with no biggy. It's not gonna tell you EXACTLY what you have, but will give you a very good figure to go by when it comes time to select a cam/stall etc. If you have an open diff (one wheel peel), you need to keep one tire still and let the other rotate two times while counting the turns of the driveline (let the pass. tire do the turning). If you have a posi rear, let both tires turn only once while counting the turns of the driveline. Before doing this, clearly mark the tire and driveline, and have the marks point straight to the ground or other easy reference point. Turn the drive line by hand, not the tire. If the drive line count is say 3 1/4 turns, then you have approximately a 3.25:1 ratio. It's pretty much common sense after you know the method.
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Old 12-22-2002, 04:04 PM
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I would go ahead with dmorris. First off you
you can see the condition of the rearend and
second you can go ahead and change that gear
oil.
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Old 12-22-2002, 04:22 PM
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If it`s a GM rear, the numbers will be stamped on the ring gear, 41 11 for example, divide them and there`s the ratio.
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Old 12-22-2002, 04:54 PM
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Or maybe, it still has an ID tag on it.
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Old 12-23-2002, 06:17 AM
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I disagree about the tire spin method. I have always been able to hit the ratio within 0.05 ratio points. Turn the tire 10 times and count the pinion rotations. You can easily estimate the pinion to within 1/10 turn.
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Old 12-23-2002, 04:30 PM
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Hey willys, never tried your method. Does it matter if it's a posi or not? I noticed BstMechs is different for an open rear or posi. I would like to try both methods when I get some free time (free time? what's that ). I guess in the past I had heard a couple of different versions, it seemed everyone had a diferent opinion. I just figured screw it and started counting teeth. Another problem with me is I'll want to try each method on a few cars just to make sure it really works and wasn't a fluke if it worked on the first one. I'm a little weird like that.
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Old 12-24-2002, 03:37 PM
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alternately, if you're just curious and don't want to take your car apart, go onto the highway, check your tach at 60, then plug the numbers into any one of the crunching programs you can find online. or follow a friend who's going a set speed, if you're not sure your speedo is right.
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Old 12-24-2002, 10:29 PM
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Yes you can do this with a posi, just make sure both tires are off the ground! If you are trying the tire spin and count method the more times you spin the tire(I have heard 30 times) and count the driveshaft the greater the accuracy, that and guesstimating to the 10th on the driveshaft, (by the way have a buddy spine the wheel while you count the revs of the drive shaft) but hey gear lube is cheap pull the cover and you will sleep easier, you may not even have to spend $2 on a gasket if you are careful!
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Old 12-25-2002, 07:42 PM
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[quote]Originally posted by Bluegrass:
<strong>I know there Is some way that you can turn the wheels and count the Revolutions of the Drive shaft

How do you tell the Ratio If you don't have the Rearend out of Truck,
turn wheel 2 turns on Driveshaft = 1-1/4 turns on the Tire
If you want to go to a higher gear ratio ---Instead of changing the ring gear in the rear end.--You can go to a tire two inches larger (Dia.) and you would go from a 3.08 to a 3.36 ratio...
[ December 22, 2002: Message edited by: Bluegrass ]</strong><hr></blockquote>
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Old 12-25-2002, 08:01 PM
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If you put LARGER tires on the rear, you will have the effect of a numerically LOWER ratio.
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Old 12-26-2002, 04:56 AM
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10 is good, 30 is better, 100 would be great! I have usually been right on with 10 turns. Posi whould work great. Only rule is that both wheels must turn on same direction. If one turns and the other doesnt turn or turns backward, ratio will be off. Posi will help in that aspect.
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Old 12-26-2002, 01:48 PM
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Exactly, thats why with an open diff you have to hold one wheel still. Through the action of the differential, it creates a 2:1 ratio, therefore you must compensate for this by turning the wheel two times or twenty to get the accuracy of the ten turn method.

[ December 26, 2002: Message edited by: BstMech ]</p>
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Old 12-26-2002, 01:56 PM
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Yep. Works every time.
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