1966 mustang rear end

As Al says, if you spend anytime on the hiway with a C4, 3.25 is about as much as you'd want to go without an OD transmission. I have a 5 speed and because I spend a fair amount of time at hiways speeds and have smallish tires (25.2), I went with 3.40 to keep my 1st gear from being useless, but even with the 5 speed, I debated whether I should go with something between 3.25 - 3.50 for quite awhile. I like the 3.40 with the 5 speed with my 3.35 1st but I might have gone with 3.50 or more if my T5 had a 2.95 1st gear. Just some things to ponder. It's all a bit of a compromise.

kenash said:

Hi,
Having run, both the 3.25 and the 3.50 with my C4, I too, agree the 3.25 is your best and final upgrade for the C4. I have since moved on to a T5 with the 3.50.
Good Luck!

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I have the T5 in my 65 coupe with 335 1st gear. I don't know yet what my rear end has in it yet. We will be opening it up in months to come. I want a good highway gear as I plan to travel a bit for summer work. I'm thinking 3.25-3.50 with Posi! What kind of economy are the T5 guys getting and how many RPM's at 65-70mph? I can't wait to see what the T5 runs like with posi rear. Are you guys running Caltraks or any traction bars? We are considering traction bars too if they are needed?
Kenash you seem to have the set up we are looking at with our engine pushing 300-350 HP and 15 inch wheels are you running into problems of wheel hop or are you running traction bars?


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Jo Scibo said:

I have the T5 in my 65 coupe with 335 1st gear. I don't know yet what my rear end has in it yet. We will be opening it up in months to come. I want a good highway gear as I plan to travel a bit for summer work. I'm thinking 3.25-3.50 with Posi! What kind of economy are the T5 guys getting and how many RPM's at 65-70mph? I can't wait to see what the T5 runs like with posi rear. Are you guys running Caltraks or any traction bars? We are considering traction bars too if they are needed?
Kenash you seem to have the set up we are looking at with our engine pushing 300-350 HP and 15 inch wheels are you running into problems of wheel hop or are you running traction bars?


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Hi,
Although my engine is rather torquey, I am not running any added rear traction devices outside of reverse-eye 4 leafs and the BFG 225X15X60s all around. These tires will break away quite easily, but, I manage the torque. As they are 25.6" my hwy Rs at 70 mph are about 1975-2K. My 5th gear is a .63.
Hope this helps.

Jo Scibo said:

I have the T5 in my 65 coupe with 335 1st gear. I don't know yet what my rear end has in it yet. We will be opening it up in months to come. I want a good highway gear as I plan to travel a bit for summer work. I'm thinking 3.25-3.50 with Posi! What kind of economy are the T5 guys getting and how many RPM's at 65-70mph? I can't wait to see what the T5 runs like with posi rear. Are you guys running Caltraks or any traction bars? We are considering traction bars too if they are needed?
Kenash you seem to have the set up we are looking at with our engine pushing 300-350 HP and 15 inch wheels are you running into problems of wheel hop or are you running traction bars?


Sent from my Autoguide iPhone

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Jo,
My bride's 66 convertible runs a close-to-stock 289 w/ 3X2's, a T-5 and 3.25 rear gears. She gets between 26-28 on the highway.
Best,
Al

I struggled with the 'something between 3.25 and 3.50' for quite awhile. I have a 3.35 1st, 215x60x15 tires (25.2) and went with a 3.40 trac-loc. If I were to do a lot of highway, I think I would have gone 3.25 but either will provide good SLR and good OD. I think there would be about 100 RPM diff or so at 70 with my .68 5th between the two.

The '66 shop manual lists 8" ratios as 2.80 (I own an original of those), 3.00, 3.25 and 3.50. The '66 GT I bought decades ago had a cracked carrier in its 8", 3.00 so at the time I bought a used Mustang 8" which had a 3.50 ratio in it with date codes of 1964. Its been too long ago to remember (if I ever knew) any other history of that 8", 3.50 except by 1982 the internals of the differential assembly were worn out and I eventually replaced everything, including the ring and pinion to return the car to its original 3.00. If anyone wants the 3.50 gearset its only taking up space on my shelf.

Looking at the Master Parts catalog it seems that 3.50 gears may have been from a Fairlane but it came to me in a Mustang differential. They might not have been originally in a Mustang but there were 8" 3.50s in the era.

ttucker14 said:

Okay so I have another question. Will a posi unit fit in the 8" that's in my stang?

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Hi again. Yes, there was a limited slip version of both the 2.80 and 3.00 gears for the 8 inch rear ends, so, there isn't any reason why that system with a different gear ratio wouldn't fit. And, yes, Ford had various gear ratios for that rear end, but, the C code Mustangs all got the 2.80 or the 2L80 rear ends and the A code Mustangs got 3.00 or 3L00. I have no problem believing that a dealership would have been happy to swap that out for something else, though, if the customer wanted it. It is a very simple matter, especially when they already have the stuff sitting on a shelf.

Veronica said:

I have no problem believing that a dealership would have been happy to swap that out for something else, though, if the customer wanted it.

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I have no idea of the history of the 8" I mentioned so it could well have been one of the 17 prior owners who put in that 3.50. :gringreen

The shop manual shows all 4 of the 8" ratios available as 'limited slip'. That's because you can put any ratio 8" gearset you want into one of those limited slip differentials. You need to realize that prior to 1969 they were not 'lockers' so if 'posi' (a nebulous, often GM term) means locking rear then the early Ford version does not qualify. They were Equa-Lok limited slip - they could still slip. It is a set of flat friction discs much like an automatic transmission clutch pack meant to withstand at least 100 ft-lb of torque before slipping. With enough torque and enough tire traction those units were no longer necessarily limited in their slippage. They are great for driving in the snow, which is why my car had EL when new, but as has been mentioned in some other threads here recently, for drag racing you might want something else.

When the clutch discs wear even the 'limited' part goes away and the rear end turns into a standard, open differential. The simple solution for that is to replace the clutches but they are no longer available from what I read. Some have resorted to substituting automatic transmission clutch plates instead. I have no idea how well those might work.

But, yes, if you don't limit yourself to an Equa-Lok there are other, non-original lockers, you could use.