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Author Topic: Question on rear end gearing/drivability/6.75" vs 8"  (Read 5156 times)

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Offline popbumper

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Question on rear end gearing/drivability/6.75" vs 8"
« on: November 16, 2013, 03:48:29 PM »
Hi all:
 
  My '76 wagon is equipped with the generic 6.75" inch rear end. A few years back, I was able to acquire an 8" rear end from a '78 glass hatch, 6 cylinder car.
  The wagon is a 4 speed standard shift. As the rear end is the ONLY part of the car that has not been restored yet (but I am going to), is it a "wise" choice to put the 8" rear end underneath, with my 2.3 standard shift? What kind of change in drivability should I expect from the different gearing? Is this a good idea, and if not, why not?
I might add, my only rationale for doing so is twofold (1) - a restored 8" rear end would be an easy swap in, and I could pull the old one out and put the new one in without having the car up on "blocks" waiting for the restore, and (2) the 8" would provide more durability SHOULD I ever opt to beef up the powerplant or swap in something else.
Your thoughts, please, thanks....
Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline bbobcat75

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #1 on: November 16, 2013, 07:14:22 PM »
The 8" is a way stronger unit but the gear ratio is really what you need to know! That would determine my choice!  A 6.75 behind a stock 2.3 will last. As long as the motor and Trans does!!!       Give us the gear ratio for both rear ends!! 

Thanks!
1975 mercury bobcat 2.8 auto
1978 mercury bobcat wagon 2.3l - soon to have a 88 tc drive line
1975 ford pinto - drag car - 2.3l w/t5 trans - project car

Offline 71HANTO

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #2 on: November 16, 2013, 07:52:58 PM »
Popbumper,
As stated, the 8" will handle much more torque and was put behind 289 Mustangs, etc. Plus, you have much wider choices of gear ratios if you ever want to change 'em. Posi units are easy to get but pricey. Finding a posi unit for a 6.75 is next to impossible. I found one for sale in 5+ years of looking. The downside of the 8 is more mass to overcome on acceleration and it weighs around 30 pounds more.
 
71HANTO
 

"Life is a series of close ones...'til the last one"...cfpjr

Offline Fred Morgan

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #3 on: November 16, 2013, 08:04:08 PM »
Charles that looks like the 8" you got from me.  lol    Fred   ;D
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Offline dick1172762

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #4 on: November 16, 2013, 09:06:15 PM »
63/4" and the 8" both use the same drive shaft. I put a 8" in both a 72 and a 80 model and didn't have to change the drive shaft. Worked for me. Chris, its not worth the trouble on a stocker unless you want a posi.
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Offline dave1987

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #5 on: November 16, 2013, 10:27:26 PM »
I put an 8" from a Mustang II in my 78 Sedan, also a 2.3L with manual trans, and had no difference in how the car performed, handled, or reacted to the swap. I also used the original drive shaft used with the 6.75" rear.

I actually did noticed an obvious difference from the swap due to the gear ratio change. The original axle was 2.73:1 and the 8" is a 3.11:1 and I have more get up and go, however the motor really races when going 65mph on the freeway now. I just don't drive on the freeway if I don't have to! :)
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Srt

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #6 on: November 17, 2013, 03:29:33 AM »
i used the old 6.75 in mine for many years (well, a few anyway_until it blew up for the last time). 
 
i tried just about all the rear gears that could be had for it at the time. 


from 3:55 to 3:40 to 3:18 (no dice on the 2:73) even a 4:?? something gear set (with posi) out of an old econoline! with a short tire that son of a gun would really keep you busy  changing gears!!!


i never had any problems with any of them in terms of performance. but for me and the kind of driving that i finally ended up doing the 3:40 was the best.


all these gear sets were taking ALL the abuse that  a turbo 2.0 could dish out.

none ever failed.



the only substitute for cubic inches is BOOST!!!

Offline 71HANTO

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #7 on: November 17, 2013, 08:52:34 AM »
Charles that looks like the 8" you got from me.  lol    Fred   ;D
Hey Fred,
Key words are "looks like".  ;)  The one I got from you still has it's original 38 year old "patina". I did get replacement axles with good bearings and backing plates to replace the one that was used as a skid plate (thanks to the tip from Wittsend). ;D I found the 6.75 posi unit after I bought the 3.40 8 from you so I may try to stick with the 6.75 for my 2.0 turbo based on SRT's comments. I have a 3.40 6.75 with bad bearings and the original low mile 3.18 rear still under my car to choose from. I'm putting it behind a  Mustang T-5 with a Camaro tail housing so I don't have to cut the tunnel. It should be fun. ;)
 
71HANTO
"Life is a series of close ones...'til the last one"...cfpjr

Offline dave1987

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #8 on: November 17, 2013, 04:30:38 PM »
I do still have a 2.73:1 carrier for a 6.75" axle, just not pinion gear/shaft for it. Also have some 6.75" axle shafts, as well as some new-old-stock pinion crush sleeves (three I think). If you need replacement parts let me know.

I like this 3.11:1 ratio, I just would like to put a T5 behind the 2.3 to get that fifth gear and make freeway driving bearable again!
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Wittsend

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #9 on: November 17, 2013, 08:09:03 PM »
Talking ratios without taking tire size (diameter) into consideration is somewhat of a moot point.  People run tires from the 50 series/13" up to the largest 16" tire they can stuff in the wheel well.  That rolling diameter can make a considerable difference in the overall tire/rear ratio results.

 Ideally it would be better to talk about engine revolutions per defined distance - say..., 100 feet to have definitive numbers to discuss.  And, even there the transmission ratios factor in for each gear.  Sadly it seems it is rarely talked about.

But, all that said for my Turbo Pinto/T-5 I opted for 3.40's over 3.00's (with 215-60-14" tires).  Under full throttle I don't think I really noticed a difference in acceleration.  But in general driving the 3.00's just put the engine in the wrong RPM range at common posted speed limits.  Thus, the drive-ability improved with the 3.40's.

Given a Pinto with say 185-70-13" or 195-60-14" tires I think 3.25's would be the best middle of the road gear for for acceleration (such as it is) and mileage/freeway RPM considerations .  Unfortunately the 3.25 was rarely used in the 8" (from Ford) and is basically an aftermarket ratio.  If the 6-3/4" is acceptable the 3.18 probably would be decent.

Dave 1987, I ran 3.00 (close to your 3.11) with a T-5.  I  can't remember what tires I used, but 5th gear at 65 MPH was around 2,000 RPM.  Even with the turbo motor it was too much for anything but level ground.  I often had to downshift to 4th gear thus negating the 5th gear.

Tom

Offline Srt

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Re: Question on rear end gearing/drivability/6.75" vs 8"
« Reply #10 on: November 18, 2013, 03:34:34 AM »
Hey Fred,
Key words are "looks like".  ;)  The one I got from you still has it's original 38 year old "patina". I did get replacement axles with good bearings and backing plates to replace the one that was used as a skid plate (thanks to the tip from Wittsend). ;D I found the 6.75 posi unit after I bought the 3.40 8 from you so I may try to stick with the 6.75 for my 2.0 turbo based on SRT's comments. I have a 3.40 6.75 with bad bearings and the original low mile 3.18 rear still under my car to choose from. I'm putting it behind a  Mustang T-5 with a Camaro tail housing so I don't have to cut the tunnel. It should be fun. ;)
 
71HANTO


i was drag racing/street racing my car heavily back then. the 3:55's were in use for that period in the cars life. the tire were always a 185/70-13 either a Continental tt714 or a Dunlop SP4 which are/were both 'short' tires. 


IIRC about 23.5" to 24"in height installed stretched over some 7" wide steel rims.(both great tires for the time) so to be honest there wasn't a huge, static, contact patch to overcome on launch from a dead stop when the tree went green.


this helped immensely with prolonging the life of the rear axle.  i ended up with the 3:40 gear set because the strip days were over and i was getting into more open road, weekend long trips.  the 3:18 didn't work because all those weekend trips could never NOT include some "up one side of the mountain & down the other" episodes. 3:18's just don't cut it at all when trying to accelerate out of one curve and into the other.
 
the car was equipped with the stock german 4spd trans which to my bank accounts dismay, didn't hold up to well to drag strip use.  i became a 'valued' customer at villa ford for replacement counter shafts & 2nd speed main shaft gears as well as countless synchro rings!!!!! (got darn good at rebuilding that sucker real quick)


to make a long story short, i beat the crap out of the old 6.75" gear sets and they never let me down.
the only substitute for cubic inches is BOOST!!!