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Author Topic: posi rear end 3.55 for 6.75.  (Read 8740 times)

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Offline americanpintobean

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posi rear end 3.55 for 6.75.
« on: September 01, 2011, 06:24:33 PM »
Can anyone help me out with this?  I am looking for a posi for the 6.75 with 3.55 or 4.11 ratio.   I just need the gearing to put in the one I have, which is non locking 3.08 ratio in a 6.75.

Offline Srt

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Re: posi rear end 3.55 for 6.75.
« Reply #1 on: September 02, 2011, 03:49:23 AM »
look into early econoline rear axle stuff
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Offline americanpintobean

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Re: posi rear end 3.55 for 6.75.
« Reply #2 on: September 03, 2011, 08:14:32 PM »
like the '60s econolines? Will a 7.5 rear end fit without any suspension modifications? 

Offline baflinn

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Re: posi rear end 3.55 for 6.75.
« Reply #3 on: September 04, 2011, 09:03:09 AM »
The 6.75 is pretty weak if you're going to be doing any kind of aggressive driving with power. Probably better to look for a rear from a V6 pinto wagon. I forget what size it is - 8" or 8.5" someone here will correct me but that will be a bolt on and gives you room to work with more power. -BruceF
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Offline dave1987

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Re: posi rear end 3.55 for 6.75.
« Reply #4 on: September 04, 2011, 08:07:07 PM »
Bruce is right, the 6.75" axle is very weak. With gear ratios like you are talking about I would assume you are using a turbo 2.3 or a V8, and the stock 6.75" axle will not last long behind one of those, even with light throttle on a V8, it's just to much power.

Again, Bruce is right, the 8" from a V6 Pinto or Mustang II would be a better choice and you would easily be able to find parts for it. I run a Mustang II 8" axle with 3.00 gears behind my stock 2.3L and stock 4spd FOG transmission and it's great. Parts are readily available to!
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Offline Srt

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Re: posi rear end 3.55 for 6.75.
« Reply #5 on: September 05, 2011, 03:03:20 AM »
i experienced great longevity and no problems to speak of in terms of durability with the 6.75 in my 71.
 
it took the abuse of street/strip drag racing & a bit of autocross/roadracing over a 'career' lasting about 60 thousand miles.
 
the ratio was 3:55. an 'open' rear end (no posi) and the ONLY thing that ever went wrong was a u-joint / driveshaft failure that bent the pinion shaft at about 50 thousand miles during a street racing contest.
 
the rear end was behind a boosted 2.0 that was not babied at all.
 
that said i was always on the lookout for a lower gear but (in those days) i didn't want the weight of a big 9" to take advantage of available ratios. not to mention the added expense of wheels/tires and what not.
 
i drove it hard but i respected it and it lasted a long time
the only substitute for cubic inches is BOOST!!!