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Old 01/16/2009, 08:22 PM   #1
GTCSMustang
 
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9-inch Rear End on Mustang GT

There has been some interesting discussion on whether Mustangs with the GT option came standard with a 9-inch rear end or an 8-inch rear end. I just looked at my 1968 San Jose built Mustang GT and it has a:

9-inch rear end
28-spline axles
WES-P axle tag
3.25:1 ratio

I'm sure it is original to the car. The car is a 302-4bbl with C-4

Scott

68.5 Blck Mustang CJ Cnvrt, 68 Lme Gld GT/CS, 68 Rd HCS (fact 428), 68 Ac Bl Mustang GT Cnvrt, 68 Ylw GT500KR Cnvrt, 68 Hlnd Grn GT500KR Cnvrt, 68 Gld G.T. 500 Cnvrt, 68 Ryl Maroon Mustang Sprint Cpe, 68.5 Hlnd Grn Fstbck, 68.5 Ylw Mustang Fstbck. Visit my website www.scottfullerreproductions.com .
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Old 01/16/2009, 10:27 PM   #2
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Scott,
Thanks for starting a new thread on this topic, I will repeat what I said on the other post. I have owned 5 '68 J-code GT Mustangs, all of them have had the 9 inch rear. Excluding my first Mustang, a '67 coupe, I have only owned '68 GT Mustangs. Did not really care about the color, options or anything else, they just had to be a factory GT.

I have also never seen a factory '68 C-code Mustang with a 9 inch only the 8 inch. I have seen numerous '68 J-code Mustang without the GT option and all of them have had the 8 inch.

These have been my observations over the years.

Danny


68 S code GT Fastback, 68 J code GT Conv., 68 1/2 R code GT Coupe




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Old 01/17/2009, 06:02 AM   #3
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I agree with the above post by 6t8-390gt. Every single 1968 Mustang with factory installed GT option that I have observed (and have owned 2 of them) has been equipped with a 9" rear axle.
In the case of the J-code and S-code GT equipped cars, the standard axle ratio was 3.25. As I stated in the previous post on this subject, the first one I owned (1968 J-code GT coupe) had a 3.00 axle ratio, listed on the Marti report as "optional axle ratio". My current car (1968 J-code GT coupe) is equipped with the 3.25 ratio. Interestingly, if you look at J-code cars that were NOT GTs, the standard axle ratio I believe was 3.00. I am unsure about what the X-code GT would have been equipped with (I have seen X-code non-GTs with the 2.79 ratio). As well, I believe that the R-code 1968.5 cars came with a 3.50 ratio as standard. There is no doubt in my mind that when you ordered the GT equipment group, you got a 9" axle. Although not stated by Ford anywhere in their factory sales literature about the option specifically including a 9" axle, it does list "heavy duty suspension" as part of the option. In my book, the rear axle is part of the suspension. There was a stand-alone "heavy duty suspension" type option for non-GT cars in 1968, but I have never seen a car equipped with this option. It would be interesting to see what one of those cars had for an axle, 8" vs. 9".
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Old 01/17/2009, 08:07 AM   #4
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I would agree that the secret is in the rear end ratio, and that part of the "GT" package seems to be a 3:25 ratio axle. (which is a nine inch) After looking through every Marti report listed on this site it appears that ALL "J" code cars with the GT package on this site have the 3:25 axle. four speed or automatic. ALL of the "C" code cars with automatic trans have the 2:79 axle. The "C" code cars with standard transmissions, either three or four speed equipped, came with the 3:00 axle. All of the "J" code cars W"ITHOUT the GT option have the 3:00 gear as well, no matter what transmission. Only one Marti report listed here shows a car with a limited slip rear end, and itis a "J" code, non GT, and it has a 3:00 axle, so there was at least one 8 inch limited slip car. These facts apply to small block cars only. No 390 or 428 information was used for this search.
Would all of you who read this and have a SMALL BLOCK car, please check the axle code on your data plate (found on the drivers door latch area) and post what you find, also telling if your car is a GT or not. I would set up a poll to make this easy but have no idea how to do that. There has been so much mystery about the 8 inch vs. the 9 inch in "J" code mustangs over the years, and the information you post here could very well be the key to understanding this oddity. I am sure that Paul N. will chime in here also, as I think he is researching this very subject for his latest book.

Last edited by murf104; 01/17/2009 at 08:43 AM..
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Old 01/17/2009, 09:03 AM   #5
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Now we're getting somewhere. Thanks Murf and teamlo and Danny.

Scott

68.5 Blck Mustang CJ Cnvrt, 68 Lme Gld GT/CS, 68 Rd HCS (fact 428), 68 Ac Bl Mustang GT Cnvrt, 68 Ylw GT500KR Cnvrt, 68 Hlnd Grn GT500KR Cnvrt, 68 Gld G.T. 500 Cnvrt, 68 Ryl Maroon Mustang Sprint Cpe, 68.5 Hlnd Grn Fstbck, 68.5 Ylw Mustang Fstbck. Visit my website www.scottfullerreproductions.com .
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Old 01/17/2009, 10:47 AM   #6
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this is interesting research.
i have a 67 gta mustang with front disc brakes and a 8 inch rear end,
and so the info is here also
my 68 gt/cs is a j code, non gt that has a 8 in rear end
seems to confirm what is said.
so then all big block cars have the gt option?

jbart
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Old 01/17/2009, 11:03 AM   #7
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Confusing, isn't it? No, all big block cars do not have the GT option, but all big block cars seem to have a nine inch. The grey area of 8 vs. 9 inch seems to be with the "J" code engines in 68 Mustangs. We hope to discover if ALL gt optioned "J" code 68 Mustangs came with the 3:25 ratio nine inch while all NON GT 68 "J" code cars were standard with a 3:00 ratio eight inch.
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Old 01/17/2009, 11:04 AM   #8
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To address JBART's question about big block cars:

Not all 390 Mustangs in 1968 were necessarily GTs. You could (at the beginning of the model year anyway) purchase an S-code 390 Mustang without having to specify the GT equipment group. I believe that the GT equipment group became mandatory with the S-code 390 later in the run (Ford sales literature updates elude to this). As well, you could get the X-code 390 without specifying the GT group.

All 428 CJ cars in 1968 (with the exception of the 50 "135 series" cars) were GTs. I believe that the standard ratio in this case was 3.50, with 3.91 or 4.30 optional.

As stated, it appears that 3.25 was the standard axle ratio on J-code and S-code GTs. My old '68 J-code GT coupe did have a 3.00 9", listed as an optional axle ratio as I stated previously (I guess for fuel economy reasons or quieter running on the interstate).
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Old 01/17/2009, 11:07 AM   #9
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My car (per Marti Report) is a 2.79 axle and sure it was an 8 inch. When we bought the car, it had a 9 in posi in the back. We restored the posi and planned to put it back in the car. The orgininal owner decided he liked the work we did on the posi and took it back. Therefore, we ended up finding another 9 in 3.25 non posi and rebuilt it. That is what is in the car today.

Paul

GT/CS sold to "PonyGal" for trip back home to California
1965 AC Cobra Replica - 390FE - Black on Black charcoal stripes - "The Black Pearl"
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Old 01/17/2009, 12:31 PM   #10
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...Maybe instead of a poll, just ask people to check & see if they have anything weird & post that. All my C-codes are 2.79ers, non-GT J-code - 3.00 and the two GT390s - 3.25.
This is a great thread and it would be neat to see if there is anything out of the norm (factory wise).

Tim
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Old 01/17/2009, 02:00 PM   #11
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OK. I've got an idea. I made a matrix (see link http://www.scottfullerreproductions....gaxleinfo.html ). Maybe with the help of people on this site we can fill this in. I'm open to comments on format too. I decided to make the matrix a link instead of posting it here so it was easier to keep track of what is current.

Please provide comments.

Scott

Last edited by GTCSMustang; 01/17/2009 at 04:23 PM..

68.5 Blck Mustang CJ Cnvrt, 68 Lme Gld GT/CS, 68 Rd HCS (fact 428), 68 Ac Bl Mustang GT Cnvrt, 68 Ylw GT500KR Cnvrt, 68 Hlnd Grn GT500KR Cnvrt, 68 Gld G.T. 500 Cnvrt, 68 Ryl Maroon Mustang Sprint Cpe, 68.5 Hlnd Grn Fstbck, 68.5 Ylw Mustang Fstbck. Visit my website www.scottfullerreproductions.com .
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Old 01/17/2009, 02:11 PM   #12
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Scott,
The matrix looks good, obviously we know the 3.91:1 and 4.30:1 were optional to the Cobra Jet.

I'll check the axle tag of my J-code GT conv. and post the info.

Danny


68 S code GT Fastback, 68 J code GT Conv., 68 1/2 R code GT Coupe




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Old 01/17/2009, 02:54 PM   #13
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Matrix updated per Danny.

http://www.scottfullerreproductions....gaxleinfo.html

68.5 Blck Mustang CJ Cnvrt, 68 Lme Gld GT/CS, 68 Rd HCS (fact 428), 68 Ac Bl Mustang GT Cnvrt, 68 Ylw GT500KR Cnvrt, 68 Hlnd Grn GT500KR Cnvrt, 68 Gld G.T. 500 Cnvrt, 68 Ryl Maroon Mustang Sprint Cpe, 68.5 Hlnd Grn Fstbck, 68.5 Ylw Mustang Fstbck. Visit my website www.scottfullerreproductions.com .
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Old 01/17/2009, 03:51 PM   #14
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My car is a NON gt and has a 3:00 rear axle, code "5" per Marti report and data plate.
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Old 01/17/2009, 05:05 PM   #15
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...So some 390s had C4s eh? I didn't know that... I always thought big block cars had the C6...

Tim
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