posi rear end gears - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Erix · 06-18-2018, 02:26 PM

My 87 Silverado is finished except for the posi rear end. I have the 350 engine with the 4 speed (overdrive) transmission that came in it. The right rear tire will light up but the drivers side doesn't. Some tech lines say 373 is best all round.I'm not racing it. Just a cruiser but would like both tires to spin and get reasonable mileage. What are other 350 guys installing that fits the bill.
Thanks,
Eric
my project can be viewed at :
https://www.fuelcurve.com/1987-chevy-c10-pickup/

*YoungPup1977* · 06-18-2018, 02:47 PM

If you have a true posi differential than I would think both tires should lay down the rubber....if you dont, you can add the posi carrier without replacing the gears...Good luck on smokin both tires !!!!!

**Captainfab** · 06-18-2018, 11:38 PM

Beautiful truck!

What gears will partly depend on what the diameter is of your rear tires and what you want out of a gear change. Are you looking for better performance or lower RPM's at highway speeds? Lower rpm's don't always mean better MPG's. The engine needs to run in it's 'sweet spot' for best efficiency.

Erix · 06-19-2018, 02:11 PM

I have 20” Mickey Thompson tires ! All I want to do is spin both tires. I can live with current gears. Thanks !!!

**Captainfab** · 06-19-2018, 11:33 PM

20" diameter tires..........what are those on 13" wheels?

Quote:

Originally Posted by Erix

I have 20” Mickey Thompson tires ! All I want to do is spin both tires. I can live with current gears. Thanks !!!

If you want a posi, then you will have to either install one or have it installed.

Erix · 06-20-2018, 03:06 AM

Quote:

Originally Posted by Captainfab

20" diameter tires..........what are those on 13" wheels?

If you want a posi, then you will have to either install one or have it installed.

Sorry !!! My wheels are 20’s on rear and 18’s on front!

toolboxchev · 06-20-2018, 03:11 AM

Get a Detroit True Track...Eaton brand.

Erix · 06-20-2018, 03:46 AM

Quote:

Originally Posted by toolboxchev

Get a Detroit True Track...Eaton brand.

Thank you very much !!

**Captainfab** · 06-20-2018, 11:40 PM

The OUTSIDE diameter of the rear tires is what is needed to determine the optimum gear ratio for the desired usage, not the wheel diameter. Your original post sounded like you were wanting a gear change also.

Quote:

Originally Posted by Erix

Sorry !!! My wheels are 20’s on rear and 18’s on front!

Erix · 06-21-2018, 03:26 AM

Quote:

Originally Posted by Captainfab

The OUTSIDE diameter of the rear tires is what is needed to determine the optimum gear ratio for the desired usage, not the wheel diameter. Your original post sounded like you were wanting a gear change also.

I’m open to change gear size if it would help but if I can posi what I have, that’s fine too. I’m trying to determine whats the best approach at this point.
Thank you for helping !

b454rat · 06-21-2018, 09:58 AM

I’d say 3.73s maybe 4.10s. With either gear the drivetrain won’t be working as hard, so tho seems backwards but would get better mileage, while having fun at the same time lol...

Erix · 06-21-2018, 10:21 AM

Quote:

Originally Posted by b454rat

I’d say 3.73s maybe 4.10s. With either gear the drivetrain won’t be working as hard, so tho seems backwards but would get better mileage, while having fun at the same time lol...

Thank you !!

**Captainfab** · 06-21-2018, 10:50 PM

What gear ratio do you currently have?

Quote:

Originally Posted by Erix

I’m open to change gear size if it would help but if I can posi what I have, that’s fine too. I’m trying to determine whats the best approach at this point.
Thank you for helping !

Erix · 06-22-2018, 12:48 AM

Quote:

Originally Posted by Captainfab

What gear ratio do you currently have?

I’m not sure. The label in the glove compartment door indicates GU2

b454rat · 06-22-2018, 09:45 AM

Believe that is 3.23.....

b454rat · 06-22-2018, 09:46 AM

Wow, it’s 2.73s.....

Erix · 06-22-2018, 12:20 PM

Thank you b454rat ! Should I just do the locker on those gears or change. If change to what & why ?

b454rat · 06-22-2018, 08:23 PM

Change the gears for sure!!!! At least 3.73s. Any posi that isn't the grenade lock.

toolboxchev · 07-01-2018, 01:18 PM

If you go from 2.73 to 3.73 gears your going to kill any mileage you got from that thing. It would dramatically change they way they whole drivetrain responds to the engine. I would bet that thing is a dog right now with those gears.

weq92f · 07-01-2018, 03:21 PM

That is a very nice looking square body right there! If you really do have 2.73 out back and a standard 700r, 1st gear off the line is probably OK, but 2nd gear must be a let down ( I'm thinking WOT here ).

If you don't want to change your current mileage/driving characteristics, don't change the gear ratio.

If you want to change the way your truck gets down the road and/or increase the fun factor stop light to stop light, change the gear ratio.

I would first make sure you know what you have currently. Determine that for certain then decide what you want to do with respect to the ratio.

-klb

Erix · 07-01-2018, 03:53 PM

The glove compartment sticker is:

06-18-2018, 02:26 PM	#1
Erix Registered User Join Date: Jun 2018 Location: Charlotte Posts: 14	posi rear end gears My 87 Silverado is finished except for the posi rear end. I have the 350 engine with the 4 speed (overdrive) transmission that came in it. The right rear tire will light up but the drivers side doesn't. Some tech lines say 373 is best all round.I'm not racing it. Just a cruiser but would like both tires to spin and get reasonable mileage. What are other 350 guys installing that fits the bill. Thanks, Eric my project can be viewed at : https://www.fuelcurve.com/1987-chevy-c10-pickup/

06-18-2018, 02:47 PM	#2
YoungPup1977 Account Suspended Join Date: Jun 2015 Location: No longer here Posts: 1,000	Re: posi rear end gears If you have a true posi differential than I would think both tires should lay down the rubber....if you dont, you can add the posi carrier without replacing the gears...Good luck on smokin both tires !!!!!

06-18-2018, 11:38 PM	#3
Captainfab 60-66 Nut Join Date: Mar 2007 Location: Coeur d'Alene, Idaho Posts: 23,246	Re: posi rear end gears Beautiful truck! What gears will partly depend on what the diameter is of your rear tires and what you want out of a gear change. Are you looking for better performance or lower RPM's at highway speeds? Lower rpm's don't always mean better MPG's. The engine needs to run in it's 'sweet spot' for best efficiency. __________________ Power Steering Box Adapter Plates For Sale HERE Power Brake Booster Adapter Brackets For Sale '63-'66 HERE and '67-'72 HERE and '60-'62 HERE and "60-'62 with clutch HERE Rear Disc Brake Brackets For Sale. Impala SS calipers HERE Camaro Calipers HERE D52 Calipers HERE 6 Lug HERE Hydroboost Mounting Plates HERE

06-19-2018, 02:11 PM	#4
Erix Registered User Join Date: Jun 2018 Location: Charlotte Posts: 14	Re: posi rear end gears I have 20” Mickey Thompson tires ! All I want to do is spin both tires. I can live with current gears. Thanks !!!

06-20-2018, 03:11 AM	#7
toolboxchev Registered User Join Date: Jul 2015 Location: 2nd left past the stump on a dirt road. Posts: 2,629	Re: posi rear end gears Get a Detroit True Track...Eaton brand.

06-21-2018, 09:58 AM	#11
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,453	Re: posi rear end gears I’d say 3.73s maybe 4.10s. With either gear the drivetrain won’t be working as hard, so tho seems backwards but would get better mileage, while having fun at the same time lol...

06-22-2018, 09:45 AM	#15
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,453	Re: posi rear end gears Believe that is 3.23.....

06-22-2018, 09:46 AM	#16
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,453	Re: posi rear end gears Wow, it’s 2.73s.....

06-22-2018, 12:20 PM	#17
Erix Registered User Join Date: Jun 2018 Location: Charlotte Posts: 14	Re: posi rear end gears Thank you b454rat ! Should I just do the locker on those gears or change. If change to what & why ?

06-22-2018, 08:23 PM	#18
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,453	Re: posi rear end gears Change the gears for sure!!!! At least 3.73s. Any posi that isn't the grenade lock.

07-01-2018, 01:18 PM	#19
toolboxchev Registered User Join Date: Jul 2015 Location: 2nd left past the stump on a dirt road. Posts: 2,629	Re: posi rear end gears If you go from 2.73 to 3.73 gears your going to kill any mileage you got from that thing. It would dramatically change they way they whole drivetrain responds to the engine. I would bet that thing is a dog right now with those gears.

07-01-2018, 03:21 PM	#20
weq92f Registered User Join Date: Oct 2016 Location: Dallas Texas Posts: 2,860	Re: posi rear end gears That is a very nice looking square body right there! If you really do have 2.73 out back and a standard 700r, 1st gear off the line is probably OK, but 2nd gear must be a let down ( I'm thinking WOT here ). If you don't want to change your current mileage/driving characteristics, don't change the gear ratio. If you want to change the way your truck gets down the road and/or increase the fun factor stop light to stop light, change the gear ratio. I would first make sure you know what you have currently. Determine that for certain then decide what you want to do with respect to the ratio. -klb __________________ 67 C10 fleet fuel injected '70 402, 700r4, 3.73 posi 07 335 sport turbo 6sp 94 Trans Am GT LT1 6sp posi -- sold after 22yrs 99 540 sport V8 6sp -- sold 73 240z L24 4sp -- given to friend 68 C10 step 350/350 3.73 open -- sold

07-01-2018, 03:53 PM	#21
Erix Registered User Join Date: Jun 2018 Location: Charlotte Posts: 14	Re: posi rear end gears The glove compartment sticker is: Attached Images

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