• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-

3.55s Or 3.73s?

  • Thread starter Thread starter Benz510
  • Start date Start date May 3, 2016

Benz510

Active Member
Feb 19, 2016
171
70
38
Oakland California
May 3, 2016
#1
  • May 3, 2016
  • #1
Hey Stangnet,

Got a question someone will probably be able to answer. I'm in the process of figuring out what the PO of my foxbody installed on the rear end and i might just have figured it out. I know for a fact they are not the stock 3.08s that came on the car. I looked at a chart for determining what rear end gears i may have. I just replaced my speedometer cable so the speedometer actually works now. The problem is i also need to change the speedometer gear in the tranny because the speedo is off, but i need to know the differential ratio obviously.

I used a GPS and at about 2300 rpms it says I'm going about 65 mph in 5th gear on the freeway. I think they're 3.73s but what do u guys think?
 

cleanLX

Founding Member
Jan 17, 2001
976
537
144
May 3, 2016
#2
  • May 3, 2016
  • #2
need to factor in tire size as well.
use this site http://www.grimmjeeper.com/gears.html to dial it in
 

A5literMan

At least it is lumpy...
5 Year Member
Jul 30, 2011
4,674
2,343
194
Illinois
May 3, 2016
#3
  • May 3, 2016
  • #3
Most likely 3.73.
 

Boosted92LX

It's only an inch or two. What's the big deal?
15 Year Member
Dec 19, 2010
5,721
4,244
224
May 3, 2016
#4
  • May 3, 2016
  • #4
Jack up the rear of the car, put it in neutral. Put a chalk mark on the inside of the tire, and on the driveshaft. Roll the tire 1 revolution, count driveshaft turns. 3 1/2 turns = 3.55. 3 3/4 turns = 3.73. This will tell you for sure.
 
Reactions: LILCBRA and FoxMustangLvr

cleanLX

Founding Member
Jan 17, 2001
976
537
144
May 3, 2016
#5
  • May 3, 2016
  • #5
another thing to consider is simply doing the math to see how far off you are as a ratio...
i offer this because, it may not be possible to tell what drive gear is installed in the trans.
I think pre 88 T5's had a 7 tooth, post 88 had an 8 tooth... autos may be different...
So, if you pull the cable and check what color gear you have, and know what % your speedo is reading "off", multiply the speedo gear tooth count by the same % and bingo... you have the speedo gear tooth count required regardless of what trans drive gear or R&P gear you have.
 

FoxMustangLvr

I love my Pimp
SN Certified Technician
Oct 14, 2012
4,968
3,514
204
Spokane, Wa
May 3, 2016
#6
  • May 3, 2016
  • #6
Boosted92LX said:
Jack up the rear of the car, put it in neutral. Put a chalk mark on the inside of the tire, and on the driveshaft. Roll the tire 1 revolution, count driveshaft turns. 3 1/2 turns = 3.55. 3 3/4 turns = 3.73. This will tell you for sure.
Click to expand...
@Benz510
Do what Allen (boosted92lx) suggested. This is absolutely the quickest and easiest way to find out what your rear gearing is. Then look up the calculator that @cleanLX suggested. It's hard to imagine that you have the correct speedo gear in your trans if you have yet to verify your rear pinion ratio unless the GPS gave you what you needed already.
 

jrichker

StangNet's favorite TOOL
In Remembrance. Thank you for your contributions
Mar 10, 2000
27,512
2,813
234
Dublin GA
May 3, 2016
#7
  • May 3, 2016
  • #7
LMR to the rescue...

to figure what speedometer gear you need - eliminates guesswork; includes all the variables like tire size.
https://lmr.com/products/Mustang-Sp...2HBaMwOReEmr4w8jRMbXio1XyQ5BfVNf5AaAkVf8P8HAQ


Now that you know what you need for the correct gear, see
https://lmr.com/products/Mustang-Transmission-Speed-Correction
 
Last edited: May 3, 2016

Benz510

Active Member
Feb 19, 2016
171
70
38
Oakland California
May 3, 2016
#8
  • May 3, 2016
  • #8
Thanks everyone. I will try boosted's method tomorrow after getting my new exhaust setup inatalled.

I appreciate it. U guys always find time to help.
 

FastDriver

I was uncomfortably high & wearing a helmet
SN Certified Technician
Sep 5, 2001
6,078
2,679
224
Vass, NC
May 4, 2016
#9
  • May 4, 2016
  • #9
Stay away from trying to calculate based on MPH and RPM unless you have an accurate tachometer. The stock tach can be off by a lot more than the ~5% difference between those gears.
 

jrichker

StangNet's favorite TOOL
In Remembrance. Thank you for your contributions
Mar 10, 2000
27,512
2,813
234
Dublin GA
May 4, 2016
#10
  • May 4, 2016
  • #10
FastDriver said:
Stay away from trying to calculate based on MPH and RPM unless you have an accurate tachometer. The stock tach can be off by a lot more than the ~5% difference between those gears.
Click to expand...
There is a narrow sweet spot for the stock tach and it is moveable by calibration. However, outside that sweet spot, there can be up to 200 RPM off. I have the electronic tools to do the calibration, and I saw firsthand what you can and can't do with an 87-91 Mustang tach. The 92-93 models supposedly have a second adjustment trimpot, but since I haven't seen one, I can't verify that.
 
Reactions: FastDriver

FastDriver

I was uncomfortably high & wearing a helmet
SN Certified Technician
Sep 5, 2001
6,078
2,679
224
Vass, NC
May 7, 2016
#11
  • May 7, 2016
  • #11
I defer to your expertise on the subject, J.
 
You must log in or register to reply here.

Similar threads

J
2004 Mustang 3.8L V6 vibrations above 65 MPH and differential leaks
  • joeybuddy96
  • Mar 16, 2026
  • SN95 V6 Mustang Tech
Replies
15
Views
491
SN95 V6 Mustang Tech Jun 28, 2026
joeybuddy96
J
D
96 get convertible-sell or part out?
  • DTCstang
  • May 17, 2026
  • What is it Worth?!?!?
Replies
5
Views
117
What is it Worth?!?!? May 18, 2026
nickyb
Drivetrain Speedo gear ate itself
  • keel
  • Aug 15, 2024
  • Fox 5.0 Mustang Tech
Replies
7
Views
712
Fox 5.0 Mustang Tech Aug 15, 2024
AeroCoupe
F
Please help 00 mustang 3.8 v6 T5 manual transmission Tail housing cracking
  • FF-DC00
  • Jan 2, 2025
  • SN95 V6 Mustang Tech
Replies
1
Views
199
SN95 V6 Mustang Tech Jan 2, 2025
General karthief
C
Help identifying 8.8
  • Caden_H
  • Jun 25, 2025
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
10
Views
628
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- Nov 12, 2025
limp
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?