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  Mustangsandmore Forum Archive
  '69 to '73 -- The Musclecar Mustang
  Best Rear Gear to Have ???

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Author Topic:   Best Rear Gear to Have ???
6T9 Sportroof
Journeyman

Posts: 78
From: Richmond,Ky,USA
Registered: Jan 2003

posted 10-23-2006 10:59 AM     Click Here to See the Profile for 6T9 Sportroof        Reply w/Quote
Currently have a 9inch 3.00 open gear that is making some bearing noise so I will be pulling it for a re-freshing and also will replace the axle seals and bearings and check balance of the drive shaft and replace the u-joints. Would a 3.25 limited slip be a good option to go back with or should I keep the 3.00 and add a limited slip to it? Would the 3.25 be that much of a improvement over the 3.00 to warrant the change? Want to keep it road worthy with manageable rpm's while running 70-75 mph on interstate.

Current set up is a 351W with 3 speed auto FMX. With present gearing, I turn 2900-3000 RPM's at 70.

Thanks

------------------
1969 Mach 1 351W-4V
1999 F250 CC 4x4 Powerstroke

Fastback68
Gearhead

Posts: 4511
From: Sucat, Paranaque, Philippines
Registered: Jul 99

posted 10-23-2006 01:37 PM     Click Here to See the Profile for Fastback68        Reply w/Quote
As they say, different strokes for different folks. For interstate driving, I think 3.00 is just right. 3:25 is more fun around town without going crazy. We don't have interstates like I found on my drive from Minneapolis to Milwaukee, so I'm a 3.25 fan. If I was doing Minneapolis / Milwaukee every week, I'd go 2.89.

SkinnyMan
Gearhead

Posts: 916
From: Tupelo, MS - USA (CA temporarily)
Registered: Jan 2005

posted 10-24-2006 12:38 AM     Click Here to See the Profile for SkinnyMan        Reply w/Quote
quote:
Originally posted by 6T9 Sportroof:
Current set up is a 351W with 3 speed auto FMX. With present gearing, I turn 2900-3000 RPM's at 70. Thanks

What tires are you running on that puppy?

Used to run '69 2V 351 with FMX and seemed to turn less RPMs than you are talking there. Of course the last time I drove mine in that configurtion was 15-years ago and the old memory banks ain't what they used to be.

------------------
SkinnyMan
Tupelo, MS

Thank you, Thank you very much!

Ryan Wilke
Gearhead

Posts: 3237
From: Stanton, Michigan, zip 48888
Registered: Oct 2000

posted 10-24-2006 09:23 AM     Click Here to See the Profile for Ryan Wilke        Reply w/Quote
6T9,

Plugging your values into this calculator;

http://users.erols.com/srweiss/calcrpm.htm

At 70mph, 3.00 rear gear, trans gear of 1-1, and 2940rpms,,,,it indicates that you're running a 24" diameter rear tire?!

Maybe your tach or speedo is off a tad or ??

Whereas, only changing the tire diameter value to 27" diameter, it calculates to only 2613 rpms.

Given a 27" tire, 70mph, a 3.25 ratio = 2831rpms.

Given a 27" tire, 70mph, a 3.50 ratio = 3049rpms.

I suggest you step up to the 3.50 with the traction-lock and don't look back. Remember: You only live once & a short life at that, you don't smile enough now, and it's only $$$$.

Ryan

6T9 Sportroof
Journeyman

Posts: 78
From: Richmond,Ky,USA
Registered: Jan 2003

posted 10-24-2006 10:44 AM     Click Here to See the Profile for 6T9 Sportroof        Reply w/Quote
After all this, I will go back tonight and measure the tire Dia. Also will jack the car up and check tire rotation's to axle rotation's to verify that I have a 3.00. Makes me second guess what I have. Door tag code says 3.00 but no tag is present on the rear end, so previous owner has had work or service done and tag got lost.

Also current tire size is 245/50x15 on a 8" Magnum 500.

------------------
1969 Mach 1 351W-4V
1999 F250 CC 4x4 Powerstroke

[This message has been edited by 6T9 Sportroof (edited 10-24-2006).]

6T9 Sportroof
Journeyman

Posts: 78
From: Richmond,Ky,USA
Registered: Jan 2003

posted 10-25-2006 07:15 AM     Click Here to See the Profile for 6T9 Sportroof        Reply w/Quote
I measured the tire and it is a 26" so that being said, I put all the values into the site Ryan linked to and it comes out that I already have the 3.25 gear.

Speed-70 Speed-70
Tire-26" Tire-26"
Gear-3.00 Gear-3.25
RPM-2714 RPM-2940

I still need to pull the stump and put new bearings and seals in and will verify the tooth count on the ring and pinion. Now I just need to figure out which posi unit to go with.

Any suggestions??

Thanks

------------------
1969 Mach 1 351W-4V
1999 F250 CC 4x4 Powerstroke

68 Coop
Gearhead

Posts: 5847
From: Mesquite, NV. 89027
Registered: Oct 2004

posted 10-25-2006 08:35 AM     Click Here to See the Profile for 68 Coop        Reply w/Quote
Detroit Locker. I found a Tarc-Lok on ebay for less than 200 bucks, got my 355's from another member for less than 200, but the install killed me at over 500.

------------------
William
M&M# 4256
MCA# 64831

68 Coupe
289
C4
3:55's/Trac-Lok

Ryan Wilke
Gearhead

Posts: 3237
From: Stanton, Michigan, zip 48888
Registered: Oct 2000

posted 10-25-2006 08:41 AM     Click Here to See the Profile for Ryan Wilke        Reply w/Quote
quote:
Originally posted by 6T9 Sportroof:
I just need to figure out which posi unit to go with. Any suggestions??

Many folks will tell ya you can't beat a good tight FoMoCo tracion-loc differential for a street/strip car. With new clutches and springs, filled it with Heavy synthetic gearbox oil and no additives, it would serve you well.

However, I've read many good reports on the newer design Detroit Truetrac differentials. They have no clutches or cones to wear out, but are gear driven.

Shop around and see what's available...
Sometimes price can be a deciding factor!

Good Luck!
Ryan

SteveLaRiviere
Administrator

Posts: 48752
From: Saco, Maine
Registered: May 99

posted 10-25-2006 10:36 AM     Click Here to See the Profile for SteveLaRiviere        Reply w/Quote
If I were buying new I'd buy a locker. It can take a beating that would make a Trac-Lock cry.

------------------
'70 Mustang Mach 1 - '70 Mustang Convertible - '72 Mustang Sprint - '94 F-150 XL

6T9 Sportroof
Journeyman

Posts: 78
From: Richmond,Ky,USA
Registered: Jan 2003

posted 10-25-2006 02:29 PM     Click Here to See the Profile for 6T9 Sportroof        Reply w/Quote
Been leaning toward the Detroit Truetrac,,,pricey at $469 plus a rebuild kit and the labor to set it up.

------------------
1969 Mach 1 351W-4V
1999 F250 CC 4x4 Powerstroke

Fastymz
Moderator

Posts: 22791
From: Reno Nv M&M #1240
Registered: Apr 2001

posted 10-25-2006 02:31 PM     Click Here to See the Profile for Fastymz        Reply w/Quote
quote:
Originally posted by SteveLaRiviere:
If I were buying new I'd buy a locker. It can take a beating that would make a Trac-Lock cry.


I love my 3.91 gears and locker! It hooks much better then the old trac lock. They aren't has noisey or harsh has some people warmed me about. Heck most of the time you don't know it's there. Until you push the loud pedal.

------------------
oddly obsessed with big scoops on little Mustangs

HOOD HACKERS DELIGHT!
My Pics

13.563 @ 108.64

[This message has been edited by Fastymz (edited 10-25-2006).]

pmibach
Journeyman

Posts: 20
From: SSF, California
Registered: Dec 2004

posted 10-25-2006 05:00 PM     Click Here to See the Profile for pmibach        Reply w/Quote
i have a 69 mach 1 and i went with the 3.50 gears and i am very happy with them

Wayne Brooks
Gearhead

Posts: 190
From: Winfield, Ks. 67156
Registered: Jan 2003

posted 10-25-2006 11:00 PM     Click Here to See the Profile for Wayne Brooks        Reply w/Quote
Very happy with my Detroit True trac. Makes absolutely no noise and hooks up much better than any Trac Lok that I have ever had.

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