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Old 12-03-2008, 10:20 AM
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Identifying what gears are in a differential?

Hey guys, I'm headed out to the junkyard this weekend looking for a new front and rearend for my truck ('84 GMC shortbed 4x4) and I'd like to know how I can tell what gears are in the rearends. I currently have 4.56's with a 700r tranny and 35's and it feels a little bit too much I'm looking for a rearend with 4.10's or 3.73's in it. I've read that everybody recommends the 14ff, but I don't have the money to change the wheels so I need to stick with a six lug. Thanks.


Last edited by muzzfactor; 12-03-2008 at 10:35 AM. Reason: forgot to state tire size
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Old 12-03-2008, 10:33 AM
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I would ask to see the inside of the diff before you buy,I would take a rachet with a 1/2 socket with you and if you find some your interested ask them if you can pull the cover or ask them to do it. The tooth count for the ratio will be stamped on the ring gear.

373 is 41/11 tooth count
410 is 41/10
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Old 12-03-2008, 12:13 PM
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If its jacked up, put it in neutral and turn the tire once around while counting the number of times the driveshaft spins. On a posi, its simple; the number of times the driveshaft turns is the ratio. Its not an exact way, but if its about 3 and 3/4 turns, its probably a 3.73. About 3 and a half would probably be 3.42. A little over 4 would be 4.10.

On an open rear (or a gov-loc that isn't engaged) you have to make sure the other tire doesn't turn, then double the turns you see at the driveshaft. Or you could use a buddy, and both of you turn a tire, then the number of turns will equal the ratio.
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Old 12-03-2008, 04:33 PM
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Some of these trucks may have the ratio on the data sticker, if it is still there.

My 1/2 2-wheeler had it on the data sticker in the door and the option sticker in the glove box. (79 C-10)

My 78 K10 had it on the option sticker in the glove box.

But---depending on where yours was built---might not have the same info.
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Old 12-04-2008, 08:08 PM
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My '75 K25 has the 14 bolt FF rear and it had a metal tag on one of the differential cover bolts. It didn't say 4.10 on the tag, but did have two numbers that calculated out to that ratio (something like 45/11).

I had some brake work done recently and the shop also changed rear differential fluid. The guy mentioned that it looked like it might have never been changed (33 years old?) - that's one of the risks you have when buying a used axle.

Most of the pick and pull places in San Antonio charge $25 if you pull a stock 8 bolt GM 16"x6.5" steel wheel.

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