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Old 04-08-2010, 05:02 PM
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Changing Differential Gears

Ok I have a 1972 C20 Chevy with 4.57's its got a granny low......that is way to low for me I am going to be putting in a 350 or a 400 in it and I am wondering what would be some good gears to put in the rear end to get me some good top end speed but I dont want to loose all my torque. Thanks.

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Old 04-08-2010, 05:38 PM
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Something in the 3.70 to 3.50 range should be good.
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Old 04-10-2010, 09:19 AM
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How much does it usually cost and is it hard to make it Posi.
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Old 04-10-2010, 11:00 AM
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Labor to change gears usually runs around $250 to $400 depending on where you are. I'm assuming that your truck has the 10.5 GM rear end, but it could be something different. I don't know what is available for the 10.5, but I can check. I know that there is a Governor Lock and Detroit Locker for it, but I don't know if Eaton makes a Posi for it. You could also have a Dana 60, which has several differentials available for it.
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Old 04-10-2010, 11:25 AM
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I know I dont have a Dana 60 I have a Eaton ....the rear end cover is completly round like a bowl.
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Old 04-10-2010, 11:45 AM
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I don't know anything about that rear end. You might want to change it over to a later model.
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Old 04-10-2010, 01:12 PM
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Search your vehicles needs here, Randy´s Ring and Pinion.
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Old 04-10-2010, 07:54 PM
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Quote:
Originally Posted by bosco200
Ok I have a 1972 C20 Chevy with 4.57's its got a granny low......that is way to low for me I am going to be putting in a 350 or a 400 in it and I am wondering what would be some good gears to put in the rear end to get me some good top end speed but I dont want to loose all my torque. Thanks.
That should be the Eaton HO52/HO72. Round back cover with a drop out third member correct? Gear sets for these are getting hard to come by.Especially a higher ratio like your wanting. At one time Precision gear was doing a 410 ratio but it seems like they might have stopped.

The common ratio for these is a 456 and 513 and some 410's. The 410 ratio is really hard to come by. You might have better luck looking for a Dana 60 out of those years then you would have all the ratio options you need.
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Old 04-25-2010, 11:42 AM
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Yea I its the Eaton HO52/HO72 like you said so I would be able to get like 3.56 around there...cuz i want to get out of the 4's.
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Old 04-25-2010, 11:43 PM
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I have a 72 c/10 pickup 350, 3.73 eaton posi 12 bolt rear end. Iwont go any lower or higher to be honest, mines great.

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Old 04-26-2010, 04:15 AM
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The '71 3/4 ton I had, was a 350/granny (compound first), with a Dana 60 (4.10). With the 28"-29" tires I had on there, 55mph (give or take a few MPH) was around 2700-2800 rpm. Not sure how accurate the speedo/tach was, and not gonna bother to do the math right now. Just reporting what I remember. Is that the kind of RPM/speed you're looking for? I'm guessing you have coils/trailing arms, and not leaf springs out back?

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Old 04-26-2010, 03:44 PM
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Quote:
Originally Posted by bosco200
Yea I its the Eaton HO52/HO72 like you said so I would be able to get like 3.56 around there...cuz i want to get out of the 4's.
You won't find one of those HO72 diff's in the ratio your wanting. Your best bet would be to look for a Dana 60 like a1supersport has.
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Old 04-26-2010, 03:49 PM
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Alright Thanks.
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Old 04-26-2010, 03:51 PM
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Sometime in the '80s the light 3/4 ton started using a 9.5 inch 14 bolt rear end that has semi floating axles. If you are just using this as a hot rod and you don't do any heavy hauling with it then this rear end might be a better way to go. These rear ends are lighter and plenty strong. There is a Eaton Posi available for them and they use 6 lug 33 spline axles. Your truck would be faster with one of these rear ends, and most of them have 3.42 or 3.73 gears in them.
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