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Discussion Starter · #1 ·
hey gang,
I have a 64 1/2 mustang that has a 260 v8, had automatic transmission.
I'm upgrading to a late 90's 5.0 and a T5 5 speed manual. My question comes to the rear differential gear ratio. I'm thinking about putting newer model rear axle in it with disc brakes and 8.8 diff. Anyone have my engine/trans combo? What gear ratio are you running? This car will be a cruiser, not a drag car. It will run 75 on the interstate, etc. 3.55LS a good choice? 3.73? I'd appreciate a little input. I'm going to research what are in newer mustangs and see what ratio fits the masses.
 

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hey gang,
I have a 64 1/2 mustang that has a 260 v8, had automatic transmission.
I'm upgrading to a late 90's 5.0 and a T5 5 speed manual. My question comes to the rear differential gear ratio. I'm thinking about putting newer model rear axle in it with disc brakes and 8.8 diff. Anyone have my engine/trans combo? What gear ratio are you running? This car will be a cruiser, not a drag car. It will run 75 on the interstate, etc. 3.55LS a good choice? 3.73? I'd appreciate a little input. I'm going to research what are in newer mustangs and see what ratio fits the masses.
Go with the 3.55s. I have the exact same combo. Some T-5s have a lower first gear - if you go 3.73 1st gear is pretty short. 3.73 will spin about 200 rpm more on the highway. I think 3.55s are the perfect compromise and the best bang for the buck (make sure you do limited slip diff too)
 

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1967 Mercury Cougar XR7
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Don't be afraid to use your 8" stock rear end. Get new gears with a Torsen type LSD, and enjoy. You don't want to run slicks and flog a T5 anyway. Street tires will not let you get enough grip to break your rear end, or your transmission, and it should work very well. They are strong, light, simple, and you can even get hardened axles for them. In a light car, they'll never die. Some guys even drag 'em very seriously. They steal less power than an 8.8 or 9" rear end, and you won't have to monkey around with welding and grinding brackets or messing with axle tube length to keep your tires in your wheel wells.

If you want to play around with tire sizes and gear ratios to see what your car's RPMs will be, visit grimmjeeper.com
You can put in your T5 for the transmission, and then play around with rear gears and tire sizes to find out what RPMs will be at any given speed, or what speed you'll go for a given RPM.

A stock(ish) 5.0 usually likes to be around 2200 rpms for max economy. Most Mustangs will run 3.55s if they see a lot of highway use, but depending on which T5 you've got (they come in a lot of different ratios) you might like 3.73s or 4.10s if you rarely get up to 75 and just goof around in town, or like tire smoke. Like @CCsDroptop (and hey, welcome to AFM, CC!) said, If you have one of the T5s that has a really low first gear, forget about anything past 3.55 because your first gear will be absolutely useless. It'll act like a granny gear in an old truck, and frankly, you're not going to tow anything with your 'Stang.
 

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8" rearend is stronger then given credit for, due to the big bro 9" over shadowing it. Go with 3.73s posi unit and new forged steel axles. The axles/bearings are old and will need replacing soon anyway. and stay away from rear disc brakes, drums on the rear work fine.
3.73s are better because of your neck of the woods. If you're in 5th gear the engine will want more RPMs to pull any hills. That means down shifting due to the final engine RPMs lugging the engine in 5th gear. Now if you where in a flat area of the USA, yeah 3.55s.
 

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Your gear size will also depend on your tire height. If you're running close to a stock size tire (about 25" diameter), 3.55 is the way to go. If you're running a bit taller than stock, then you can get away with a 3.73.
 

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Discussion Starter · #6 ·
Thanks for all the input. This car will likely get a fair amount of highway use w/65mph limit. Some on 70 limit. Its not going to be a tore burner. Just a nice driver. Not putting huge tires on rear , just something similar diameter to stock. Probably going with 16" wheels, maybe wider than the original but more for looks than traction. I do not know the stock ratio in it now. It had an automatic, not a c4. Its a big cast iron cased automatic transmission. I assume gear ratio is fairly fast due to limited trans ability. Maybe a 3.00?? I figured it it needs bearing kit, ring and pinion, axles, and maybe a limited slip setup, just upgrading to a newer properly geared axle would save time and money. As far as brakes, I'm certainly not against drums, just think disc are a little better.
This car is stripped from stem to stern, rear axle is still in it but I'm debating on whether I keep the leaf springs or put a coil over setup in the rear. I've seen a couple nice coil over setups.
I'm a truck guy, not really into mustangs, but this one is for my wife. Just trying to build a nice classic driver that is modernized tastefully. Not making a Sema car, nor do I have the budget to. But I'm not afraid to cut and weld and put good improvements into the car. I want it geared good for all around driving and I don't want it running 3500rpm at 80. Lol that gear calculator will help me decide on that as well.
I will consider all advice, I appreciate you all taking the time!
 

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Surprisingly, drums in the back have a lot of advantages. They are lighter than most disc setups, and with good pads, stop even better. If you're road racing or autocrossing, you'd want discs, because after several high speed stops in a row, the drums can get overwhelmed. They don't shed heat as well. But for normal use, they really do work great, and often give better ride quality thanks to their lower weight.
Coil springs add some complexity, but usually do not improve ride quality enough to justify the cost of the 3 or 4 link setup along with all the reinforcements over a good set of leafs. A set of Bilstein shocks will give you far more bang for the buck.
In your search for betterment on a budget, I think you would be a lot better served by upgrading the internals on that rear diff and spending your money on some StreetOrTrack adjustable roller perch front control arms, 1" front sway bar, and adjustable strut rods that mount solidly to your frame. Then get a set of Kelsey-Hayes 4 piston front discs to give yourself more 'whoa' and a set of good high-friction pads for all around. (most modern pads are junk, now that they don't have the original friction materials in them - because asbestos actually worked well, except for making people die.)

Most of the earlier Fords with automatics got 2-speed cast iron Ford-o-Matics, and probably the most common axle ratio was 2.79:1 I am not sure that the Ford-o-Matic was a normal transmission for the Mustang though - I thought all of them got the 3 speed aluminum cased C4!
 

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I’ve tired 2.73, 3.31, 3.73 in my ‘95 with a T5. 2.73’s were stock and they were ok I guess with the stock cam, but it wasn’t worth a darn with an aftermarket one. It would buck and chuck at low rpm’s. Then I went to the 3.73’s because that’s the forums said was “the best.” Well, it felt like driving a tractor, and with stock power levels it was still painfully slow. During regular driving I’d be shifting into second within ten feet of taking off and I would be in 5th gear between stop lights. Under full throttle runs I would have to shift to 3rd to do 60 mph and 4th ran out about 110 or so (2.73’s would do 70 mph in 2nd and 130 mph in 4th at around 6k rpm). If I was cruising on the highway around 70 mph in 5th it would engine brake if I let off of the gas. The 3.73’s were pretty tiresome to be honest. So I went to a 3.31 which is what is in there now. They’re nice, it feels more refined if that makes any sense. More like a “regular car” I guess. It might accelerate a tad slower but not enough to worry over since it’s not a race car. However, it’s not as easy to drive in parking lots as it was with the 3.73’s. If I had a stock cam it prob wouldn’t be an issue though. If you’re just cruising around I’d go with a 3.27/3.31. Maybe a 3.55. All that being said there’s not a rear end out there that’ll make a 200 horsepower car fast or anything close to fast.
 

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Discussion Starter · #9 ·
Yeah this car is a 64 1/2, it is a 260 car. Auto trans is cast iron. The guy got it from was going to put a c4 in it, I have that also. But its not staying automatic, or 260. I'd keep the 260 but it is a 5 bolt bell pattern. T5 won't fit it without adapter plate that cost more than its worth. So I have bought the 5.0 to build and use. Not going for foolish power. Just a nice running nice sounding car.
I haven't tried that gear calculator yet but I will. Just want to find the best ratio to fit the widest range of our use. Its not going to race, drag or otherwise. I don't want it to be a dog either, I may have to school a camaro on occasion. Haha
I haven't made a decision on the rear axle and brakes yet. In my opinion disc brakes are an upgrade over drums. I'm not against drum brakes, but there's a reason most new cars are built today without drums. Disc has proven to be better. But I'm not concerned about brake choice here.
I'm sure my current axle has an open diff, and likely a ratio i don't want. So aside from a re-gear, it needs a limited slip carrier too. I can find 8.8 axles around here for a good price usable as is, although I'd at least put a bearing kit in. Last 8.8 I bought for a f100 project cost me $75 with a 3.73 limited slip out of an explorer. I see them regularly under $200. So that is a viable option.
As for springs, there are some nice coil over kits out there, or I can stay stock. Not sure im stuck on one way or the other. Basically, I don't want this car to drive like its a 65. I'd like it to drive like a much newer car, and just look old.
 

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Not to put too fine a point on it, but I don't have a horse in this race either way. The 8.8 is a good axle too, but it will be challenging to narrow one to fit your car. It will involve a lot of cutting and welding to remove the original bracketry, and add appropriate mounts for your car, but far more daunting will be the width, as that will require very precise measuring, cutting and welding for the axle tubes, as well as custom length axles. The early Mustangs are narrower than any of the 8.8 rear ends.
I won't disagree that discs are 'better' than drums. In fact, in the front, they're critical! But the reason they're used in modern cars for the rear is as much about perception as it is about performance. There are excellent reasons why they were used for so many years in the back end of cars. Many people discover that a cheap set of disc brakes in the rear lead to slightly harsher ride quality and worse braking - not better - compared to a properly set up pair of original drums.
And having tried many cars with fancy coilover kits that cost an awful lot, it's remarkable how a few careful changes can typically get you 90% of the way to a kit that costs you ten times as much. This is especially true for rear suspension, although I freely admit that if cost were no concern, I'd probably put in IRS for my own car. For the most part, live axle is live axle, and changing all the mounts and springs for it does not matter when you're shaking a few hundred pounds of unsprung weight up and down there in the back of your car. You don't have to take my word for it though. Get in touch with any honest and reputable suspension company. I would recommend StreetOrTrack, and talk to Shaun!

Real world experience is sometimes surprising.

Anyway, best wishes, and hope you love the choices you make!
 

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Yeah this car is a 64 1/2, it is a 260 car. Auto trans is cast iron. The guy got it from was going to put a c4 in it, I have that also. But its not staying automatic, or 260. I'd keep the 260 but it is a 5 bolt bell pattern. T5 won't fit it without adapter plate that cost more than its worth. So I have bought the 5.0 to build and use. Not going for foolish power. Just a nice running nice sounding car.
I haven't tried that gear calculator yet but I will. Just want to find the best ratio to fit the widest range of our use. Its not going to race, drag or otherwise. I don't want it to be a dog either, I may have to school a camaro on occasion. Haha
I haven't made a decision on the rear axle and brakes yet. In my opinion disc brakes are an upgrade over drums. I'm not against drum brakes, but there's a reason most new cars are built today without drums. Disc has proven to be better. But I'm not concerned about brake choice here.
I'm sure my current axle has an open diff, and likely a ratio i don't want. So aside from a re-gear, it needs a limited slip carrier too. I can find 8.8 axles around here for a good price usable as is, although I'd at least put a bearing kit in. Last 8.8 I bought for a f100 project cost me $75 with a 3.73 limited slip out of an explorer. I see them regularly under $200. So that is a viable option.
As for springs, there are some nice coil over kits out there, or I can stay stock. Not sure im stuck on one way or the other. Basically, I don't want this car to drive like its a 65. I'd like it to drive like a much newer car, and just look old.
The 5.0 HO and T-5 combination was used by Ford in umpteen thousands of Fox body Mustangs using the 8.8 geared at 3.08. Even my '93 Cobra.
 

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If it was a '67+, changing to an 8.8 might make more sense, but the extra cost and effort involved with reworking the axle to make it fit won't make sense compared to just putting different gears and LSD in his 8". Even if you throw in the cost of disc brakes being put on the 8" (which is a questionable addition for most street cars anyway), it's probably going to be a wash, pricewise.

The T5 might be a great choice, as far as driveability goes, with a mostly stock 5.0.
 

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Gears and a limited slip diff are pretty easy to come by cheaply for the 8". I think I paid $150 (shipped) for a really good condition used 3.55 ring & pinion set with a Traction-Lok LSD off of ebay. I've got a center section out of a '73 Mustang (more structural ribs) out of the local pick-a-part for $50, and thru some horse trading, I have a whole rear end out of a '66. My biggest problem now is finding the time to clean, paint, and assemble it all, and get it into the car.
 

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8' ratios
2.79
3.00
3.25
3.40
3.50
3.55
3.80
4.11
4.62
The last two are aftermarket only. The 3.40 and 3.55 were in Mustang IIs. The 3.80 ( once again available in the aftermarket) was originally in an Econoline and hard to find. A "new" traction lock ( billet material) is good or a true track or Detroit locker Stay away from a Spartan , EZ Locker and other that use stock side gears and cases.
 

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3.73 still my choice. OD = .67 of final gear. AKA 3.73=2.48 gear. 3.55=2.36 gearing. To much of a drop unless you have a light car or 300+ honest HP. It doesn't look like much. But the engine will know the difference. As for 1st gear in the trans, personal choice. Ford even made the toploader in close ratio or wide ratio.
 

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@rickyracer1983,

Besides the wide and close ratio "production" top loader gear ratios , Ford had about twenty special "DSO" gear ratio sets for the top loader. These "white stripe" transmissions as they were called , were used in Nascar , Trans Am and LeMans cars ( in the Ford trans axle). They featured gears with bronze bushings in them for increased endurance in racing applications. First gear ranged from a stout 1.76 to a low 2.93. I still have some of the gears to make a 2.14 - 1.54-1.19 trans used in the '69 T/A Boss 302 program. Ford did lots of things the
"general public" knew nothing about.

Randy
 

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How much effort getting them into a ‘65? Is the juice worth the squeeze? The T5 is good but I dunno if it’s worth reinventing the wheel if that’s what it takes.
My '65 K model came from Ford with a 9-inch rear, 3.89 gears, Top Loader 4 speed, I think 2.78 first gear. Enough '65 and '66 Ks were built that it might be possible to find one in a bone yard still having it's rear axle intact. THAT would be well worth using.

I put a 9-inch in my '89 Fox by welding appropriate suspension bracketry onto it. 9-inchers were much more common beginning in '67, but the tread distance (track width) was wider. Narrowing one would be not easy for the casual user. Several aftermarket outfits specialize in 9-inch axles (Strange, etc.) and that might be a route to follow.

As far as the T-5 goes, it would be no problem installing one in a '65.

My error, above, K cars came with 2.32 first gear, 2.78s were available.
 

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The 2.78 wide ratio was not available with the K code engine , only the 2.32. I was there in the day.
Several aftermarket companies make "bolt in" 9" housings and axles for the '65-66. Quick Performance is one that does. For anything less than serious drag racing , the 8" is plenty adequate.
Randy
 

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I had a 66 with a built 289 that I put a T5 on with an 8.8 fox body conversion… I’d say 3.55 is great for this setup on a cruiser. Still plenty of get up. For comparison for modern cars tho, I had a 2005 GT with 3.55 gears and traded for an 09 GT/CS which came with 3.73 gears as part of the package.. same motor but I def noticed an improvement in acceleration, didn’t notice much difference on the highway, and was never paying attention to the fuel economy 😋
 
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