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Old 04-16-2018, 10:41 AM   #1
85/Silverado
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Posi Rear End

I want to switch my 10 bolt stock rear gear to a slip posi. and was wondering how many teeth are on the rear axles so I get the correct Differential (30 or 33)? The one that's in the truck is a 2.73. and with my motor I get no traction so I'm hoping the posi will help.
Should I go for the Posi 2.73? I only get 8 mpg as it is.

1985 Silverado
383 Stroker 425HP+
TH 350 B&M shift
12" wide rear tires
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Old 04-16-2018, 11:09 AM   #2
nonstop
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Re: Posi Rear End

Factory rear I believe is 28 spline. Only way to know for sure will be to pull an axle and count, as who knows what’s been changed in 30+ years.

As for gearing, you may want to actually consider stepping to a lower gear. Depending on cam and engine specs, you could be below the engines power band. You could actually see an increase in mileage with lower gears.
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Old 04-16-2018, 11:17 PM   #3
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Re: Posi Rear End

If your 383 is a true 425hp, you definitely need a lower gear than 2.73. You're never going to get good mpg's with 425hp pushing a brick down the road, but you should be able to get over 10 mpg.
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Old 04-17-2018, 04:20 PM   #4
85/Silverado
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Re: Posi Rear End

maybe a 3.73?
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Old 04-18-2018, 12:33 AM   #5
Captainfab
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Re: Posi Rear End

Possibly. It depends on how your engine is built. What cam is in it? Do you have a stall converter? What size are your rear tires?

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Originally Posted by 85/Silverado View Post
maybe a 3.73?
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Old 04-18-2018, 09:18 PM   #6
85/Silverado
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Re: Posi Rear End

Rear Rims 18x9.5"
Tires 235/50R18
2500 stall converter on a new TH 350 B&M shift kit tranny.
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Old 04-19-2018, 12:18 AM   #7
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Re: Posi Rear End

OK, your tires are only 27.3" in diameter and that cam is somewhat mild for a 383. I would probably go with a 3.07 gear. That will keep your rpm's reasonable at highway speeds plus it should help with the mpg's and possibly help a little with the traction issue. Of course a posi will also help with the traction issue. That would be my recommendation. You could go with 3.42 gears, but that is going to raise your highway rpm's a bit.
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Old 04-19-2018, 12:16 PM   #8
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Re: Posi Rear End

Thanks Cap.
I loved your town when I lived in Spokane. Used to go down under the freeway bridge and jump off the cliffs. Fun days in 80 to 84.
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