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rear end gear
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71chevy
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PostPosted: Tue Feb 06, 2007 3:13 pm    Post subject: rear end gear Reply with quote

O.K. Here is what I have! 350sb/350 turbo According to smokemup calculator my rear end gear is 4.018:1. I desprately want a higher gear end ratio as @ 50 mph I'm already @ 2500 RPM. I absolutely do not race. This is strictly a drive around town/express way and maybe pull the jet-ski on sunday truck! Now that being said, give me your recommendations!
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clay
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1972 Chevrolet Nova

PostPosted: Tue Feb 06, 2007 6:51 pm    Post subject: Reply with quote

Assuming it isn't a lock up 350, and factoring in a little converter slippage, my closest guess would be you have either a 3.73 or a 3.90 gear. From what you are telling me your usage is, a 2.73 or 3.08 would be a better choice. If you go aftermarket, the highest shown in Jegs is a 3.08 (assuming you have a 1/2 ton pickup with the 10 bolt "corporate"). If you could find a 4 wheel drive shop in your area, you might be able to get a 2.73 or 3.08 from them as a takeout for next to nothing as they need to be changed to a lower gear when large tires are mounted.
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71chevy
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PostPosted: Tue Feb 06, 2007 8:31 pm    Post subject: rear end gear Reply with quote

Actually it's a 12 bolt. What do you mean by a "lock up", excuse the ignorance. Thanks for the info!
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af2
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PostPosted: Tue Feb 06, 2007 8:38 pm    Post subject: Reply with quote

I was thinking 3.08 also!
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10sec.et
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PostPosted: Tue Feb 06, 2007 8:58 pm    Post subject: Re: rear end gear Reply with quote

71chevy wrote:
Actually it's a 12 bolt. What do you mean by a "lock up", excuse the ignorance. Thanks for the info!


lock up is kinda like a poor mans overdrive. the converter "locks up" via electric solenoid when in third gear at cruising speeds making the trans gearing 1:1. if i didnt explain that correctly then someone correct me.

my vote is for the 3:08 also

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beersngars
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PostPosted: Wed Feb 07, 2007 12:00 am    Post subject: Reply with quote

Maybe I'm missing something, but what is your tire diameter???
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71chevy
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PostPosted: Wed Feb 07, 2007 11:00 am    Post subject: rear end gear Reply with quote

The tire size is 275/60/15. To get the tire diameter I simply measured from the top of the tire to the concrete slab that the truck is sitting on and got 27 in. Is this the correct way to obtain it.
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clay
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PostPosted: Wed Feb 07, 2007 6:28 pm    Post subject: Reply with quote

That's as good as any. That's the same size I run on my Nova. 27" to 28" is close enough. The truck 12 bolt stuff might not be as common as the corporate 10 bolt stuff if you are looking for used factory stuff. Clay
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71chevy
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PostPosted: Wed Feb 07, 2007 8:32 pm    Post subject: rear end gear Reply with quote

Do you know if there is such a thing as to put a 2.83 gear into a 12 bolt gm rear end????????
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71chevy
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PostPosted: Wed Feb 07, 2007 9:59 pm    Post subject: rear end gear Reply with quote

I meant 273
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clay
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PostPosted: Thu Feb 08, 2007 3:42 am    Post subject: Reply with quote

I would guess one is available for the truck 12 bolt from the factory. Try looking around on E-bay and see what turns up. Might be a little harder to find stuff for a truck 12 bolt than a passenger car 12 bolt (the car is version is more desirable from a pure strength standpoint - ironcally it is stronger than the truck version due to a larger pinion shaft) but it should still be available but probably a little more expensive. Clay
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71chevy
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PostPosted: Thu Feb 08, 2007 8:13 am    Post subject: rear end gear Reply with quote

Would 2:73 gears from a 10 bolt fit the 12 bolt correctly?
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71chevy
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PostPosted: Thu Feb 08, 2007 8:36 am    Post subject: rear end gear Reply with quote

Me again. I found a 12 bolt truck 2:73 gear out of 4wd. Would it fit into my pumpkin?
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85GMCstreetbeast
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PostPosted: Thu Feb 08, 2007 10:46 am    Post subject: Reply with quote

Yes!
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Big Dave
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PostPosted: Thu Feb 08, 2007 4:25 pm    Post subject: Reply with quote

I would hang around that four wheel drive shop and find somebody wanting a stiffer gear and swap the whole rear end. The two series gear requires a two series carrier or a special thick ring gear to fit, or a spacer to fill the void when using the wrong gear series with the wrong carrier. The twelve bolt has a separate carrier for the two series gears, the three series gears and a four series carrier for all the rest of the gears (4.10, 4.56, 4.88, 5.13, 5.38, all the way up to 7.30).

By swapping the whole rear end you avoid having to set up the rear end (expensive and time consuming). You can keep your brakes and axles if you want by pulling the rear cover and popping the C-clips out and unbolting the backing plates; or just rebuild what you get with a fresh brake job. I guarantee a drum brake job is cheaper than a rear end set up.

The 8.5 corporate rear end is actually stronger than a truck rear end but the truck rear end has bigger axle bearings for load carrying. If you are swapping a car rear for the truck the car brakes will probably be undersized in capacity compared to the truck. Even the standard duty light truck will have heavier duty brakes than what they slap on a hot car of the same year (Corvette excluded).

Problem you will find is that most modern vehicles equipped with OD trannies will have higher numerical gearís than you are looking for. You will need to center your attention on eighties trucks with a TH400 tranny for a four wheel drive or a TH350 if it was a 305 equipped truck.


Big Dave
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