Posi Rear End

[85/Silverado]

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

[nonstop]

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.

[Captainfab]

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.

[85/Silverado]

maybe a 3.73?

[Captainfab]

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?

Originally Posted by 85/Silverado

maybe a 3.73?

[85/Silverado]

Rear Rims 18x9.5"
Tires 235/50R18
2500 stall converter on a new TH 350 B&M shift kit tranny.

[Captainfab]

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.

[85/Silverado]

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.