Rearend Gears / Limited Slip vs Open Diff

Help Support 7173Mustangs.com:

Darren 72

Well-known member
Joined
Jul 14, 2010
Messages
230
Reaction score
1
Location
NH
My Car
1972 Mach 1
Hey Guys,

I have a few questions.  I'm rebuilding the 9" rearend on my 72 Mach.

First question who has experience with 3:50's or 55 gears.  Going back and forth from the 3:25's or the 50's or 55's?  I noticed with the calculator that you can find on line isn't a heck of a lot of difference in RPM's between them @ certain MPH.  How is highway with 50's or 55's?

Second question has anyone had any issues running limited slip?  Mine is open and thinking of going the limited slip route.

Thanks,

Darren

 

Darren 72

Well-known member
Joined
Jul 14, 2010
Messages
230
Reaction score
1
Location
NH
My Car
1972 Mach 1
Thanks...will take a look at this.

 

timachone

I love my nostrils!
Joined
Jul 10, 2011
Messages
1,893
Reaction score
424
Location
Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
Hey Guys,

I have a few questions.  I'm rebuilding the 9" rearend on my 72 Mach.

First question who has experience with 3:50's or 55 gears.  Going back and forth from the 3:25's or the 50's or 55's?  I noticed with the calculator that you can find on line isn't a heck of a lot of difference in RPM's between them @ certain MPH.  How is highway with 50's or 55's?

Second question has anyone had any issues running limited slip?  Mine is open and thinking of going the limited slip route.

Thanks,

Darren
Hi Darren, 

I have an original 351C 4V with 4-speed, 9" rear with 28-spline axle and 3.50 gears, open diff. I will change to limited slip and bought this 5 years ago from a different vendor: 

https://www.cjponyparts.com/ford-performance-differential-trac-lok-limited-slip-28-spline-9-/p/M4204F28A/

I do not know anybody who was unsatisfied with this, everyone is happy about it! 

Here is a good article about the procedure with the change 

https://www.diyford.com/mustangs-disassmble-rear-axle-rebuild-assembly/

They have also some good other articles about this topic... I will follow these instructions this year and will report on my resto-thread. 

As I can remind (it's almost 6 years ago) my car ran on 3.000 rpm with 3.50 gears in 4th gear on 60 mp/h and 275/60/15 tires... For me that was ok. I drove 5 hours on the highway on 60 - 70 mp/h and never had a problem. It "hang" really good on the gas (had 281.5 HP on the rear wheel until it died) in every gear and I would never change that. I like how it was in the good ol' days, four on the floor and 3.50 gears. I think 3.50 is snappy enough but still tolerable for today's traffic purposes. 3.73 and 3.91 would be another story but hey, if I had a Boss I would also change nothing about it... Long story short: 3.50 will be still fine to take out for a longer ride. 

Hope that helps a bit... 

 

droptop73

VIP Members
Staff member
Moderator
Joined
Jul 5, 2010
Messages
6,516
Reaction score
264
Location
Grand Rapids, MI
My Car
73 Convertible
Hi Darren, I went with a 3.55 gearset and Yukon gear Trac lok from Quick performance. I agonized over which way to go on the ratio between 3.25 and 3.55. Now that I have a couple of years and many miles on the set-up I am happy with the 3.55 under most conditions. My main issue with the ratio I chose is around town cruising between 45 and 55 MPH. The RPMs are a bit higher than I'd like in 4th gear and it lugs a little in 5th. This certainly will vary from car to car based on transmission gear ratios and tire size. If I were to do it again... for MY car... I'd go with the 3.25. It would also bring the RPM down a bit on the highway. Don't disregard a 1-200 RPM change it does make a difference. 

I did have to trim a little bit off the pinion support boss for clearance with the Yukon carrier. 

 

jspears

Active member
Joined
Aug 3, 2018
Messages
29
Reaction score
3
Location
USA
My Car
73 Mach 1
I think the truetrac is probably the best. Its like having a spool under throttle and an open diff at cruise. 

I was running 2400 rpm with an FMX (1:1) and kind of big tires, 2.75 rear. Then went to 235/60R15 and my RPM at 60 went to 2500.

Now i'm running a .63 final gear with 4.11 and I'm at 2200 RPM at 60mph.

Interestingly the tires I have are supposed to be 26.3 inches tall but I have to put in 24.5 to make the calculator work.

I would say you would get about 2950 with an auto and 3.25 gears at 60 and 3180 from the 3.50's?

 

Hemikiller

Tech Advisors
Staff member
Moderator
Joined
Jul 12, 2010
Messages
3,580
Reaction score
796
Location
Killingworth, CT
My Car
71 Mach 1
65 coupe
3.50 is an excellent all around gear for street performance, and it's highway "liveability" will depend on your rear tire size. A 28" tire is completely different than a 26" or 25.5". I ran a 3.40 gear with 26" tires for many years and was very happy with the highway manners.

If you cruise at 3000 with 3.50s, you'd turn 3043 with 3.55s, hardly even worth thinking about. 

As for which limited slip unit to choose, I went with a Powertrax Grip Pro in my new 9" build. It's a helical gear unit like the TrueTrac and both are highly regarded. The clutch TracLok units work fine too, but decided to go this way. 

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,115
Reaction score
876
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
I think the truetrac is probably the best. Its like having a spool under throttle and an open diff at cruise. 

I was running 2400 rpm with an FMX (1:1) and kind of big tires, 2.75 rear. Then went to 235/60R15 and my RPM at 60 went to 2500.

Now i'm running a .63 final gear with 4.11 and I'm at 2200 RPM at 60mph.

Interestingly the tires I have are supposed to be 26.3 inches tall but I have to put in 24.5 to make the calculator work.

I would say you would get about 2950 with an auto and 3.25 gears at 60 and 3180 from the 3.50's?
After changing axle gears did you get the correct size speedometer gear? Have you compared your tachometer with another one?

 
Joined
Jan 7, 2015
Messages
8,032
Reaction score
490
Location
Western North Carolina
My Car
Multiple Mustangs!
I have couple mustangs with the For Traction Loc and no issue. As long as you do not put synthetic grease in and add the friction modifier you are good to go. The next rear I build will probably be 4.10 since for a track days car and will have a T-5. There are a bunch of NASCAR 9" rears for sale on Facebook Market place I posted links the other day on here. Some even have the pumps in them for rear grease cooler.
Now a note on the traction loc. If you go to youtube and watch the idiots doing burn outs leaving car shows and loosing it they all have posi or traction loc. A guy here crashed his Viper hit a dip in the road right when he banged another gear and the rear broke loose and he went through the side of a building. Got is fixed and lost it again so fixed and sold it. Another friend in Wisconsin had Viper for company car. He went to a Viper driving school. They started out in Dodge Neon and learned the track and then got in the Vipers. You had to give them a credit card with $30,000 on it to drive the Vipers. He could never turn a faster lap in the Viper than he did the Neon due to the posi traction.
Last year at a show a woman had her brand new Roush Mustang as she was leaving she tried to do a burn out hit a light pole and totaled the car.
If you are going to drive one hard for sure get in an open parking lot and learn where it breaks loose.
In my 1984 20th. Anniversary mustang it has T-5 with traction loc. We were sitting in the inside lane on the 6 lane in Myrtle Beach was first car at the light and red. I always watch my rear mirror when stopped to be sure nobody hits me. The car right behind me stopped but I saw the headlights of the next car were not slowing down. Crossing traffic just feet away in front of us. I revved the engine dropped the clutch and jerked the steering wheel and made the car spin 180 deg. around the front of the car beside us on the right. Wife, son and daughter in the car and they thought the crashing car had hit us but I out ran it. Totaled the car behind me and the one that did not stop.
A Detroit locker is even worse to drive and way noisy. All the drift cars had Detroit lockers and sound like they are breaking doing slow tight turns.

 

Don C

Fords Forever
Joined
Jan 1, 2012
Messages
8,115
Reaction score
876
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
Mine has Detroit Locker and a hard shifting C6 that chirps the tires when shifting. Accelerating around a corner when it shifts from 1st to 2nd will cause the rear end to lock and jump out, how much depends on how heavy my foot is and how slick the road surface is. I know what it's going to do and am ready for it. I'm hesitant to let my wife drive it for that reason, will likely put in a calmer shifting 4R70W transmission when it goes back together.

David is correct, learn the idiosyncrasies of whatever rear end you put in, and make sure that anyone who drives it knows how to handle it.

 

Spike Morelli

Well-known member
Joined
Apr 12, 2015
Messages
479
Reaction score
160
Location
Boise, Idaho
My Car
1971 Mustang Mach 1 ram air 351c H-code, fmx, ps,pb, medium yellow-gold, hubcaps and beauty rings.
I run a 3.50:1 Trac-Loc in my Mach. I installed the gears mainly for more grunt to tow with. I also run the stock 14" wheels. Rpm at appx 65/70 is 3500. I don't mind it, however more than one person riding passenger has commented on the RPM. I guess it comes down to what you're used to.  

 

jspears

Active member
Joined
Aug 3, 2018
Messages
29
Reaction score
3
Location
USA
My Car
73 Mach 1
After changing axle gears did you get the correct size speedometer gear? Have you compared your tachometer with another one?
I bought a kit from american powertrain and everything was wrong, including speedo gear. I am waiting on a new engine install and maybe new wheels and tires... then rear gears and probably converting to an electric speedo.

 
Top