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Old 11-30-2009, 09:28 PM   #1
GMCMG
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Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

Don't laugh if this is a stupid question, but will the ring/pinion gears from a GM 12-bolt 8.875" car rear end fit in a GM 12-bolt 8.875" truck rear end? I am trying to achieve good gas mileage, without swapping complete axle/leafs/etc.
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Old 11-30-2009, 09:31 PM   #2
bhoeger70
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

No, the carrier is the same if I am right, BUT the pinion bearing is larger on the car 12 bolt. The truck pinion bearing is the same diameter as the 8.2" car 10 bolt.
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Old 11-30-2009, 09:31 PM   #3
1968SWBBigBlock
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

Nope = pinion bearings and a few other key components are the wrong diam
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Old 11-30-2009, 09:45 PM   #4
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

Any ideas how I get something around 2.41 gears in my 78 truck axle? Or am I just stuck finding a car axle to stick under it? (I'm going for mpgs here)
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1964 GMC 1/2 ton LWB project
- 1982 GMC 6.2 diesel engine/suspension donor
- 1966 Chevy 3/4 ton frame/doors/fenders donor
- tons of parts - cause you can't build a truck without 4 of everything!

2001 3/4HD Duramax Diesel extended cab lwb4x4
1984 Chevy C30 6.2 diesel lift truck (project)
1962 GMC w/5 lugs, 350/350 auto (was for parts, but couldn't bring myself to do it!)

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Old 11-30-2009, 10:51 PM   #5
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

A while back there was a guy on the Racing and high performance forum here that put a set of passenger 12 bolt gears in his truck 12 bolt. He said that he found the pinion bearing to make it work. I don't remember his user name, but you might try a search or a post over there.

Here I just found the thread.....
http://www.67-72chevytrucks.com/vboa...ight=car+gears
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Old 12-01-2009, 01:28 PM   #6
cableguy0
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

Believe it or not using a 2.4x gear doesnt automatically mean you will get better mpg. your really going to be lugging the engine around town just to get the truck moving with that gear. chances are on the highway your rpm will be out of the engines powerband causing it to lug and causing you to keep your foot in the throttl just to keep moving down the road. i'd hate to see you go through all the trouble of doing this to find out your mpg gets worse. what gears do you have now?
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Old 12-01-2009, 07:13 PM   #7
PhilaTruck
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

I'm with Cableguy. I just swapped from a 2.73 and I -think- my mileage has gotten better because I'm in the cam's zone.
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Old 12-01-2009, 07:56 PM   #8
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Re: Can I fit 8.875 12-bolt car ring/pinion in truck rear end?

Cableguy0 / Philatruck,
I really don't have anything in it right now. This is a build, and I'm starting from scratch. The 6-lug 10-bolt that's under my frame is a 3.54, and I think the 5-lug 12-bolt in the donor truck is a 3.73 (haven't opened it yet). I've got a 2.73 ring/pinion that will fit the 10-bolt, but it doesn't do me much good since I'll be swapping out to the 12-bolt. I was hoping to pick up a set of car gears to go to 2.56 or 2.41, but I think you guys have talked some sense into me. I'm dropping a 6.2 diesel in the truck, and using a 5-speed OD tranny with a .73 final ratio. I was hoping that with the torque the diesel can put out, that I could run low rpms, but I may be shooting too low. After reading your responses, I ran some calculations based on larger tires and 2.73 gears (something a little more rational), and I think I'll be able to pull close to 65 or 70 at 1600-1700 rpms, which is really probably all the lower I should plan on running the engine. I think (?) I can get truck 2.73 gears for the 12-bolt, so I may start there. I'm doing a frame-off, so I really wanted to get everything right the first time, as I won't be able to try it out until it's pretty well done, and this is my first project. Thanks for the input!!! I have had a lot of help in a short time from members here on my various questions.
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1964 GMC 1/2 ton LWB project
- 1982 GMC 6.2 diesel engine/suspension donor
- 1966 Chevy 3/4 ton frame/doors/fenders donor
- tons of parts - cause you can't build a truck without 4 of everything!

2001 3/4HD Duramax Diesel extended cab lwb4x4
1984 Chevy C30 6.2 diesel lift truck (project)
1962 GMC w/5 lugs, 350/350 auto (was for parts, but couldn't bring myself to do it!)

glascock.m@hotmail.com
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