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It seems a lot of mustangs have 3.55 rear ends. Mine is 3.50 according the the shop manual and there is no 3.55 listed. I know there is little difference between 3.50 and 3.55 rears, but which one is correct for a 8" rear?
 

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8" could have the 3.55 available. The 3.50 is a 9" rear end gear ratio. This is a stock Mustang 3.50 rear end gear? What year and do you have the tag info off your 8" rear end?
 

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3.55 is typically aftermarket gears. I had a 62 Fairlane that had 3.50 gears. The differential I just put in is 3.55 with Richmond gears
 

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8" could have the 3.55 available. The 3.50 is a 9" rear end gear ratio. This is a stock Mustang 3.50 rear end gear? What year and do you have the tag info off your 8" rear end?
As a comparison the 9" has a 3.70 option and the 8.8" has a 3.73.
 

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I installed an aftermarket 8" in my '66. Currie supplied the 3.55 unit.
 

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It seems a lot of mustangs have 3.55 rear ends. Mine is 3.50 according the the shop manual and there is no 3.55 listed. I know there is little difference between 3.50 and 3.55 rears, but which one is correct for a 8" rear?
You could get a 3.50:1 rear gear in both the 8 and 9-inch axles. The 3.55:1 is an "8-inch only" ratio.

For the 8-inch rear end, the 3.55:1 ratio wasn't used, to my knowledge, until the 8-inch was installed in the Pinto/Bobcat/MustangII.

FWIW, there are ALSO differences in gear sets even in the same ratio! For instance, the OEM Ford as well as "better quality" gear manufacturers 3.55 rear gear uses a pinion with 11 teeth and a ring gear with 39 teeth. The "budget" brands use a pinion with 9 teeth and a ring gear with 32 teeth. Obviously, the greater number of teeth the more surface the load is spread out upon. Also, generally speaking, the pinion with the greater number of teeth will be larger in diameter (with a corresponding ring gear that is "thinner" to compensate).
 

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I have always thought the 3.50:1 was for 8" and 3.55:1 was for 8.8". Anyone with further insight on this?
 

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I have always thought the 3.50:1 was for 8" and 3.55:1 was for 8.8". Anyone with further insight on this?
First use of the 3.55 I can find is the 71 Pinto with the 6 3/4" rear.

First use of a 3.55 axle in an 8" was the 74 Mustang II and later in the Pinto/Bobcat 8", as Woodchuck noted. No 9" axles from Ford through 79 came with the 3.55 gear that I can find.

FWIW, "3.55" is a buzz word that became popular with the 5.0 Mustang crowd, kinda like 3/4 race cam with SBC guys. 3.55s and 3.73s were common 8.8 axle upgrades since the majority of the Fox bodies came with 2.73 or 3.08 rears.
 
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I snagged my factory 3.55 8" rear end out of a Pinto wagon from the bone yard in the 80's. They doubled the price from $50 to $100 when they found out it was a T-loc. :)
 

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I have a 3.55 in my Ford 8 inch as well, I think it’s a fairly common aftermarket setup and runs great with a T5!
 

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Discussion Starter #13
The reason I asked the question is because so many people have 3.55 and I seemed to be the only one with 3.50's. The table in my 67 shop manual lists no 3.55 options, only 3.50.
 

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The reason I asked the question is because so many people have 3.55 and I seemed to be the only one with 3.50's. The table in my 67 shop manual lists no 3.55 options, only 3.50.
I have 3.50's in my '68, matched to a C4. 3.55's in my '67 matched to an AOD. I did them this way so to be able to find the right gearing for the speedometer cable.
 

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Ive never understood Ford gear ratios. I have a 2.79. Isnt there also a 2.80? Whats the difference?
 

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Ive never understood Ford gear ratios. I have a 2.79. Isnt there also a 2.80? Whats the difference?
The 2.79:1 ratio uses a 8-inch ring gear with 39 teeth and a 14-toothed pinion.
The 2.80:1 ratio uses a 7-3/4-inch ring gear with 42 teeth and a 15-toothed pinion.

The difference in ring gear diameter is to accommodate the larger pinion.
 

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According to the wikipedia, the Ford 8.8 has a wide range of ratios offered, 2.26, 2.47, 2.73, 3.08, 3.27, 3.31, 3.45, 3.55, 3.73, 4.10 & 4.56, 5.14 I had a '95 F-150 that came with a 3.27, but the tooth count was the same as the Ford 3.31. Could be that they ran out of 3.27 gears and slipped in a 3.31 but never changed the vin tag.
 

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The only way to know the true ratio is to count the teeth on both and divide.
 
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