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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 10-26-2012   #1 (permalink)
ttucker14 is offline Rookie


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Smile 1966 mustang rear end

Hello all, I was just wondering what rear end comes in older mustangs. I've done some research and I believe mine has an 8". So my question is, one day if I decide to do a gear swap could I swap something in like 3.50's or 3.73's and what would be a good gear ratio for my stang? I just drive it around town and occasionally on the highway. It has a c4 in it if that matters.
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Unread 10-26-2012   #2 (permalink)
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Undoubtedly, your Mustang came with a 2.785, 2.80, or 3.00 rear gear ratio. The next step would be a 3.25, which is about all the ratio I would recommend. When you move up to 3.40 or 3.50 gears, you are turning a lot of RPM's at highway speed, which is annoying.
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Unread 10-26-2012   #3 (permalink)
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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.
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Unread 10-26-2012   #4 (permalink)
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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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Unread 10-27-2012   #5 (permalink)
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i got the same set up as kenash. very happy cruise anywhere at anytime with good fuel mileage
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Unread 10-27-2012   #6 (permalink)
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I got 3.80s and a posi an it's a blast
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Unread 10-27-2012   #7 (permalink)
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Quote:
Originally Posted by AKBray907 View Post
I got 3.80s and a posi an it's a blast
As do I, but without the AOD I have, I wouldn't spend much time on the freeway. Even with the AOD (14" tires), I'm about 2400-2600 rpm to keep up with fast lane traffic.
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Unread 10-27-2012   #8 (permalink)
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3.89's and she's windin' pretty good at highway speeds. The launch is pretty awesome though.
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Unread 10-27-2012   #9 (permalink)
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Quote:
Originally Posted by sp66stang View Post
i got the same set up as kenash. very happy cruise anywhere at anytime with good fuel mileage
Hi,
Yup, great combo!
Happy Trails!
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Unread 10-29-2012   #10 (permalink)
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Quote:
Originally Posted by kenash View Post
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!
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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Unread 10-29-2012   #11 (permalink)
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Quote:
Originally Posted by Jo Scibo View Post
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.
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Unread 10-30-2012   #12 (permalink)
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Quote:
Originally Posted by Jo Scibo View Post
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,
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
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Unread 10-30-2012   #13 (permalink)
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Hello. In the 65/66 Mustangs there were only 2 gear ratios available for the 8 inch rear end, 2.80 and 3.00. For the 9 inch rear ends there was 3.50, 3.89 and 4.11. On the integral carriers used on the 6 cylinder cars there was 2.83 and 3.20.
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Unread 10-30-2012   #14 (permalink)
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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.
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Unread 10-30-2012   #15 (permalink)
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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.
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