Need rear gear recommendations

[prayers1 153]

Been a long hiatus, hello old friends!

Need help deciding rear gear ratio.
Have a 69 with a 408 Cleveland (525hp) an AOD transmission, 10” converter, 2800 stahl. 9” rear with 4:11. Can’t drive far (1 hr) because the motor is revving so high and tons of drone noises. IIRC, 60 mph @ 2800 -3000 rpms. Tire-295/50/15 26.6 dia.
Like to be cruising 2500 @ 65 mph.
Thinking 3:50 gears or 3:75.
Thank you for helping!

 

[Mach1 Driver 560]

Hello again, and dang I love that color. An AOD overdrive gear ratio is 0.67:1? Many people prefer a 3.5:1 rear end. Play with a calculator like this to give you an idea:

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

[det0326 179]

Hello prayers hope all is well and welcome back. Most all the AOD's from factory had a 355 or 373. I had a 373 in my 69 with AOD and it ran around 2500 rpms at about 60mph. A drone is usually caused by your muffler/exhaust system the higher the rpm's just amplifies it so you may want to look into that as well.

[RPM 1,190]

Welcome back prayers! You certainly have one of the better looking 69s. I was happy with 3.50s when I had an aod in it. I currently have a .78 OD and 3.25 which I also like.

[prayers1 153]

Thanks for warm welcome. 
Losing a wife of 33 years brought me down some horrible roads, but through it all I held onto the Mustang. Took it out a few times last Summer. I would love to go on cruises but the 4:11’s are killing me and makes for an uncomfortable ride. 
 

I understand the exhaust drone, but at a lower rpm I can tolerate it.

So, are you guys thinking more towards the 3:50s vs. 3:73.  I don’t think 3:73 is too far from 4:11s

Thanks again!

[Mach1 Driver 560]

Yeah I've been there and done that, as you know. Time helps, but more than anything, when the time is right for you, find another person to share your life with. Life can be sweet again; maybe bitter-sweet, but sweet.

I still think you should try the speed/gear calculator. If I remember correctly my ol' Vette (2001) when in OD is around 2k between 60 and 65.

[prayers1 153]

I’ve been recently remarried. Your right bitter sweet. Not a fan living alone.

I’ll take the car out today and see exactly where I’m at at 60 mph

[det0326 179]

the higher the rear end ratio the more low end torque you will loose. With 525hp u may want to loose some tho. LOL

 

[prayers1 153]

Just got back from a ride. 
60 mph @ 2400 rpms

[rwcstang 201]

might be something to think about, But Im putting down 460 rwhp 475rwtq in my 408w and i have a TKO600 with .064 and 3:50's at 65 mph, Im around 2k.

[aslanefe 333]

My quick calculations says with 3.75 you will do 66mph @2400 rpm and with 3.5 you will do 70 mph @2400. 

[prayers1 153]

Thank you aslanefe!!

I kinda lean towards the 3:50 with those calculations. These days, racing just isn’t as fun as it use to be. Now a days you really need to lean into it racing these modern cars and the crowd of cars everywhere’s isn’t worth the liability.

im thinking it’s more fun to cruise and better drivability on long highway runs.

my only concern is if the trans and Transmission and torque converter (stall 2800) can handle the 3:50’s. 
 

Also, I’m gonna get a 3rd member built with whatever gear I decide to pick, this way I still keep the 4:11s to switch if needed. So… should I build it with a Posi or open rear? 

 

[Mach1 Driver 560]

I would put in an Eaton Truetrac- try Quick Performance. The EATON Truetrac uses helical gears instead of clutches to perform its limited-slip function.

[det0326 179]

As nice as your car looks I personally would not race it, I would be treating it like a baby. If you never intend to race or do any mud slinging an open rear would be my choice. If you ever want to race again take the 411 and build a race car around it. 

[aslanefe 333]

3 hours ago, prayers1 said:

Thank you aslanefe!!

I kinda lean towards the 3:50 with those calculations. These days, racing just isn’t as fun as it use to be. Now a days you really need to lean into it racing these modern cars and the crowd of cars everywhere’s isn’t worth the liability.

im thinking it’s more fun to cruise and better drivability on long highway runs.

my only concern is if the trans and Transmission and torque converter (stall 2800) can handle the 3:50’s. 
 

Also, I’m gonna get a 3rd member built with whatever gear I decide to pick, this way I still keep the 4:11s to switch if needed. So… should I build it with a Posi or open rear? 

 

I had 2:73 on my 70 grande with C6 with 7.25 inch diff. Take off was not impressive but I could burry the speedo needle and would still have plenty rpms left.

Put in 3:25 last year, take off is still not impressive, has hard time breaking loose the rear tires with brand new fresh rubber. This is on a stock 289 (yes 289, not 302).

Only reason I put on 3:25 is, lost oil on 2:73 and it started whining; had access to an other complete 7.25 with 3:25 gears. So swapped the whole rear end.

If you want to cruise (and highway driving) I would say go with 3:50. I am not an expert but don't know why trans can't handle it, you might need a convertor with lower stall speed though.

[det0326 179]

my only concern is if the trans and Transmission and torque converter (stall 2800) can handle the 3:50’s.

The higher the gear ratio the more wear it will put on bands and clutch frictions but 350 will not be a problem in your car. Lot of explores came with 355 gears and AOD I even had a 4 wheel drive Bronco with 355 gears and 32 inch tires but of course it had an E40D trans. 350 gear will be a good selection for your car. I'll bet the 525HP Cleveland won't hardly know the difference.

[prayers1 153]

I was also concerned about any I’ll effect on the Transmission, torque converter vs Cam and rear gear ratio.

The trans has been well built to hold 500hp. The Cam is specs are:

6O2/574, 235/241 @50, love sep 112.

The Cam was built around the TC and 4:11s

What do you guys still  think?

Drove yesterday, tach must be off 100-200 rpms 60 mph @ 2200 rpms

 

 

[det0326 179]

16 hours ago, prayers1 said:

I was also concerned about any I’ll effect on the Transmission, torque converter vs Cam and rear gear ratio.

The trans has been well built to hold 500hp. The Cam is specs are:

6O2/574, 235/241 @50, love sep 112.

The Cam was built around the TC and 4:11s

What do you guys still  think?

Drove yesterday, tach must be off 100-200 rpms 60 mph @ 2200 rpms

 

 

Hey John

A cam grind has a specific RPM power band. The stall converter allows u to hold brake or trans brake in some cases to reach the lower end of that RPM band so that u are already in your specific RPM band when u release trans brake. Unless u are racing u will not know the difference with the 2800 stall. Stall converters are streetable up to around 3500 the only thing the stall converter may do is maybe use a tad more fuel but with 525HP I don't think u would notice that either.

[prayers1 153]

Ok. Thank you for explaining that. I really appreciate it!

[det0326 179]

1 hour ago, prayers1 said:

Ok. Thank you for explaining that. I really appreciate it!

it gets a lot more involved and complicated in drag racing of course but that is a simple explanation.

[prayers1 153]

Contacted PTC , who built my converter and was told stall is 3000-3500. 
All along I always thought it was 2800.

Well he said 3:55 would be ok.

[Rich Ackermann 174]

On 5/21/2022 at 10:38 AM, prayers1 said:

Thank you aslanefe!!

I kinda lean towards the 3:50 with those calculations. These days, racing just isn’t as fun as it use to be. Now a days you really need to lean into it racing these modern cars and the crowd of cars everywhere’s isn’t worth the liability.

im thinking it’s more fun to cruise and better drivability on long highway runs.

my only concern is if the trans and Transmission and torque converter (stall 2800) can handle the 3:50’s. 
 

Also, I’m gonna get a 3rd member built with whatever gear I decide to pick, this way I still keep the 4:11s to switch if needed. So… should I build it with a Posi or open rear? 

 

I have a 70 Mach with a 408 Cleveland with a Tremec TKO 600. I am happy with my 3.70 gears in a Yukon case and the Tremec with a .64 overdrive and a 27.83" tire diameter gives me about 1,850 RPMs at 65 Mph. Although from a stop first gear does not get me very far, before I am shifting into 2nd gear.  I have an Eaton TrueTrac and I love it. As Mach1 Driver said, the helical gears behaves much like an open rear until you get on it and then it locks without delay. No noise.

 

 

[rwcstang 201]

On 5/20/2022 at 11:03 AM, prayers1 said:

Thanks for warm welcome. 
Losing a wife of 33 years brought me down some horrible roads, but through it all I held onto the Mustang. Took it out a few times last Summer. I would love to go on cruises but the 4:11’s are killing me and makes for an uncomfortable ride. 
 

I understand the exhaust drone, but at a lower rpm I can tolerate it.

So, are you guys thinking more towards the 3:50s vs. 3:73.  I don’t think 3:73 is too far from 4:11s

Thanks again!

go with 3:50's, I have him them on my 69 with a 408w and a tko600 with 64 OD. at 65mph Im slightly around 1800-2k rpm. putting down 460rwhp with 470rwtq, plenty of of power with the 3:50s and OD makes it all quiet and enjoyable. 

[potato 10]

Ive got 3:50 rear with a T5 in my car.  It'll do 60 no problem.  But highway speeds around here is 80mph.  Around town, everybody does 60 on every road.

 

I find 3.50 gears to be too much.  I'd rather have closer to 3.0.  Definitely would want anything more than 3.50 for a cruiser car.

[prayers1 153]

Went with Mach1drivers suggestion, helical gears. I went with the 3:50s