1964 Thunderbird rear end gearing

[s1nemesis1s]

Hello all!

I wanted to pick your brains a bit. I have a 64 an I am going to rebuild the rear end in it myself. I was thinking about changing the gear ratio. Is anyone running something other than stock gears? I believe mine is 3.00 - was thinking about going to 3.25 or 3.50 or somewhere in there? 3.73 seems like it might be too aggressive. I am just looking for aid in low-end torque, decent enough cruising experiencing - I know RPMs will go up, and of course, better tire burning abilities :D I am looking to build the rear strong - any kits or sets that are stronger than stock anyone is running? In the end, she is all show and a cruiser, but I would like her to get out of her own way off the line and lay down some good rubber.

[RedBird64]

Yes onthe 3.0 ratio. SInce we don't have over-drive, these cars spin pretty good at highway speeds.
The diff is pretty stout, it's the famous 9" Ford.
I had mine gone through many years ago and when the mechanic talked to the Ford parts guy, the Ford guy got exceited and told him it was a Nodular Iron unit - a rare and even more robust center section.
Parts are readily available, even disc brake conversions. Stronger axles are available through several vendors including Strange Eng. Adding LSD would be a great upgrade with lower gearing.

Personally, the gihway experience is "busy" enough (my wifes words) as it is but my dream setup would be to replace the trans with a Ford AOD trans and 3.73 gears. Not only would you have tire shreading gears but you'd still have higher overall gearing for the highway!

Scott

[s1nemesis1s]

Wow, thanks! I was hoping to keep the cruise o matic if I could. I'm wondering how that would be with 3.73 gearing? Maybe I should go mid 3's on the gearing? LSD is something I will look at real hard as well. But let's say I go mid 3's and keep the cruise o matic - am I looking at a disaster lol?

From using online calcs and of course checking manually with 3.73 I seem to add around 500RPMs from 15-65 (main speeds I will going. I hope I'm not too off on that. Thoughts?

Thanks guys!

[jtschug]

I have a radial tire that has about the stock tire diameter: 27.3 inches. The manufacturer (Firestone) says this tire turns 764 times per mile.

With 3.00:1 rear, this converts to an engine RPM of 2483 in 3rd gear @ 65 MPH

Switching to 3.73:1 gives 3087 in 3rd gear @65MPH , so almost exactly 600 more RPM If you go up to 75MPH it is at 3562 RPM, which is really screaming for a bone stock 390. Red line is about 5500RPM, and if you dropped to 2nd gear in a kickdown you'd be at 5200RPM @ 75MPH

3.55:1 gives you these:

2938 in 3rd @ 65 MPH
4289 in 2nd @ 65 MPH

3390 in 3rd @ 75 MPH
4950 in 2nd @ 75 MPH

I've modified my car with a C6 transmission out of a '66 428 donor car. I've purchased but haven't installed yet a Gear Vendors Overdrive that bolts to the back of the transmission and provides a 0.78:1 overdrive gear that drops down the engine RPMs.

So with OD, the 3.55:1 will give me:

2291 in OD @ 65 MPH
2644 in OD @ 75 MPH

Yes, the nodular iron case is better than the grey iron case. The other thing to consider is noise. Some aftermarket gears are noisier than others. Loud rear axle gears sound like a jet taking off. If you can, get old original Ford gears, as long as they aren't worn, those are the quietest.

[RedBird64]

With 3.00:1 rear, this converts to an engine RPM of 2483 in 3rd gear @ 65 MPH

Yep, that's exactly what my EFi screen shows at 65.

S

[s1nemesis1s]

I have a radial tire that has about the stock tire diameter: 27.3 inches. The manufacturer (Firestone) says this tire turns 764 times per mile.

With 3.00:1 rear, this converts to an engine RPM of 2483 in 3rd gear @ 65 MPH

Switching to 3.73:1 gives 3087 in 3rd gear @65MPH , so almost exactly 600 more RPM If you go up to 75MPH it is at 3562 RPM, which is really screaming for a bone stock 390. Red line is about 5500RPM, and if you dropped to 2nd gear in a kickdown you'd be at 5200RPM @ 75MPH

3.55:1 gives you these:

2938 in 3rd @ 65 MPH
4289 in 2nd @ 65 MPH

3390 in 3rd @ 75 MPH
4950 in 2nd @ 75 MPH

I've modified my car with a C6 transmission out of a '66 428 donor car. I've purchased but haven't installed yet a Gear Vendors Overdrive that bolts to the back of the transmission and provides a 0.78:1 overdrive gear that drops down the engine RPMs.

So with OD, the 3.55:1 will give me:

2291 in OD @ 65 MPH
2644 in OD @ 75 MPH

Yes, the nodular iron case is better than the grey iron case. The other thing to consider is noise. Some aftermarket gears are noisier than others. Loud rear axle gears sound like a jet taking off. If you can, get old original Ford gears, as long as they aren't worn, those are the quietest.

Wow! Thanks! So wondering if it's worth it - don't want to run my engine ragged, but I would like a bit more a kick. 3.2ish area maybe - what do you all think of that? Would it be a noticeable difference? Did a quick google search and turned up with these - https://www.amazon.com/Motive-Gear-F890 ... merReviews

https://www.quickperformance.com/Ford-9 ... 20417.html

https://www.summitracing.com/parts/ugr- ... /overview/

[jtschug]

Yes I think 3.25 would be noticeable. These are heavy cars and a some extra spins help get them moving.

'Worth it' is really up to you. How much time do you spend on the interstate going 75 MPH? Are you on mostly 2-lane blacktop going below 60? Stop and go traffic?

If you want to burn rubber and cruise around town and almost never go on the highway, you could consider 3.50, but I would definitely upgrade the differential to a limited slip of some kind (positrac, locker, truetrac, etc) so you could leave two black stripes like a boss, not just one.

While it isn't a 1:1 ratio, more engine RPMs means fewer miles per gallon. I drive mine less than 1000 miles a year, so I don't think about it much, but I'm not getting better than 10 MPG with my stroked 428, 500HP engine, and it is still sluggish off the line, so I am in the midst of putting a higher gear ratio in the back.

[s1nemesis1s]

Yes I think 3.25 would be noticeable. These are heavy cars and a some extra spins help get them moving.

'Worth it' is really up to you. How much time do you spend on the interstate going 75 MPH? Are you on mostly 2-lane blacktop going below 60? Stop and go traffic?

If you want to burn rubber and cruise around town and almost never go on the highway, you could consider 3.50, but I would definitely upgrade the differential to a limited slip of some kind (positrac, locker, truetrac, etc) so you could leave two black stripes like a boss, not just one.

While it isn't a 1:1 ratio, more engine RPMs means fewer miles per gallon. I drive mine less than 1000 miles a year, so I don't think about it much, but I'm not getting better than 10 MPG with my stroked 428, 500HP engine, and it is still sluggish off the line, so I am in the midst of putting a higher gear ratio in the back.

Omg 428 stroked 500hp....it is literally making my mouth water. That is so awesome! Let me know how your gears work out!

The car will tool around town mostly, I may take it on the interstate for 30-50 minutes once in a while - to a local meet but that will NOT be often. It might see maybe 1000 miles a year, maybe 2000 - 1500 under 60mph.

The car will never be a daily driver or take long road trips and it is a tribute to my mother who passed away May 22nd at 12:15 pm - and I do not want it stolen or destroyed. She loved Mustangs, and this is not a mustang, but she liked these cars because they aren't "big boats," as she would call the other cars lol. She thought they were a luxury sports car :) Bless her soul...I miss her so so so much...I am so glad I was able to be there for her during her 3-year fight...I sold my home to help with her EOL care...I watched her go...hardest thing I ever saw (only 61 when she passed...).

Phew - sorry about that - Anyways, yes I want to burn some rubber for sure. What would be involved with a posi (etc) rear end? would new Axles be needed? I am thinking hard about the 3.50 gears. I am want to keep my cruise-o-Matic. I will rebuild it next - trying to collect all the info I can on them - found a nice manual for 5 bucks online from oldmanuals.com or something like that. Then I will move up to the motor. I will probably keep it stockish - maybe an RV cam and oversized pistons. Maybe just an overhaul. I am trying to work my way from the back to the front. Just wondering if 3.50ish or maybe between 3.20ish and 3.50ish the answer.

These are all firsts for me. However, I have many tools, have the support of YouTube, manuals, and all your great people- I think I can do this.

Thoughts? - Thank you all by the way.

[jtschug]

The ring gear bolts to the differential. The diff is what needs to be replaced to get posi, or whatever you choose. Pick the limited slip diff you want and you'll see how it goes in there. The '64 has 28 spline axles, you will need to pull out the axles to remove the center section to swap the gears. When you do, you should change the axle seals. Make sure the axles are smooth where they touch the seals, sometimes they get rough or worn. You may want to change the wheel bearings, but you will need to take it to a shop with a press to do that.

[s1nemesis1s]

The ring gear bolts to the differential. The diff is what needs to be replaced to get posi, or whatever you choose. Pick the limited slip diff you want and you'll see how it goes in there. The '64 has 28 spline axles, you will need to pull out the axles to remove the center section to swap the gears. When you do, you should change the axle seals. Make sure the axles are smooth where they touch the seals, sometimes they get rough or worn. You may want to change the wheel bearings, but you will need to take it to a shop with a press to do that.

So, I can keep my housing, this would just be the work inside the "pumpkin" for the posi? Should I upgrade the axles or do you think 3.20ish - 3.50ish along with posi be okay for the stock 28 spline? I have the axel seals on hand. Our anniversary is on the 6th - wife bought me a 20-ton press off Amazon! It is coming soon so I should be able to press the bearings out and in myself:)

Again - any suggestions and feedback is most welcomed!

https://www.amazon.com/gp/product/B00FG ... UTF8&psc=1

[RedBird64]

Wow S1, She's a keeper for sure!!
(Your wife that is) ;)

Scott

[jtschug]

The thing that kills axles is traction. Axles start breaking when you put on drag slicks and start dropping the clutch. Automatics are easier on axles because the torque converter provides some cushion between the engine and the wheels. So if you are sticking with stock 390, MX trans, and street tires, you are probably good with 28 spline axles. I have a 31 spline axle out of my 428 parts car, the downside is it uses a weird 9 3/8” ring gear so the whole 3rd member must be replaced to change to posi and a new ratio.

Things get expensive real fast and it is easy to get carried away. If you aren’t drag racing the car, I’d stick with the 28 spline axles and save the money for other stuff, maybe a nice gift for that generous wife

[s1nemesis1s]

Wondering if I should go with a solid spacer or a collapsible one. I have read pros and cons just wondering what you all think?

Also - I love my wife, she is amazing to support me the way she does in this hobby - baking parts dry in the oven and all. I am going to get her some diamond earrings for our anniversary, she has never had a pair - excited to give them to her! Saved for a long time!

[s1nemesis1s]

So, I am eyeballing this: https://www.amazon.com/gp/product/B0078 ... UTF8&psc=1

I am very close to buying for overhaul.

I am also considering - gulp - this: https://www.amazon.com/dp/B0078U9TAM/?c ... _lig_dp_it

Lastly - https://www.amazon.com/Yukon-Gear-F9-35 ... ive&sr=1-1

What do you all think?