Rear differential upgrades?
Printed From: The Ford Torino Page
Category: Model Specific Forum
Forum Name: 1968-1969 Ford and Mercury
Forum Description: Technical discussion for 1968-1969 Ford and Mercury
URL: https://forum.grantorinosport.org/forum_posts.asp?TID=21772
Printed Date: 26-March-2026 at 2:22PM
Software Version: Web Wiz Forums 12.06 - https://www.webwizforums.com
Topic: Rear differential upgrades?
Posted By: mrbpatsfan
Subject: Rear differential upgrades?
Date Posted: 01-August-2025 at 2:36PM
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Hello fellow enthusiasts. My 68 Torino GT came with a FE 390, C6 transmission, and an open differential with a 2:73 GR. The GR is way too conservative. I’m thinking limited slip with 3:25 through 3:73 for around town with 15” tires. Any why nots based on performance? Thanks in advance. ------------- X |
Replies:
Posted By: Jaybee
Date Posted: 08-August-2025 at 2:00PM
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A 3.55 is usually a pretty good compromise between RPM on the highway and good getaway torque on 15" car tires, as opposed to tall sidewall truck tires. With those, sometimes you need to go steeper. ------------- Cars are meant for driving |
Posted By: 72 RS 351
Date Posted: 08-August-2025 at 2:50PM
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The 390 has plenty of torque to work with taller gearing than a smaller engine. I had 3.25 gears in my first car with a C4 and 351C-4V. That was not too much gear for me at all, I think I'd try the 3.50 gear unless you need lots of highway driving time. I have 373's in my 1998 Explorer, and in 3rd gear(1:1) it runs 3000rpm at around 60mph. Your car will take shorter tires such as 26-27" I'd guess, so the same gearing should push yours over 3000rpm @60mph. ------------- Don 73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000 rpm 351 stroker 73 Ranchero GT 351C-4V &4R70W for sale later. 92 Lincoln Mark VII SE GTC, OBDII 347/4R70W |
Posted By: 72FordGTS
Date Posted: 08-August-2025 at 6:40PM
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I agree with the others. The 3.25 is a nice all around gear, 3.50 better for someone who doesn't do a lot of highway driving but wants lots of acceleration. ------------- Vince 1972 Ford GTS Sportsroof - Survivor, One Family car GTS.org Admin |
Posted By: Rockatansky
Date Posted: 09-August-2025 at 3:59AM
what performance level are you feeling? you don't mention highway at all but comments all reflect the inevitability of at least some highway time happening. your 390 should not be a slouch in the TQ dept with a 3.78" stroke, that attribute can be used to run a slightly lower numerical gear than say a 289 with a (can't get off it's ass) 2.87" stroke. have you had a play with a gear ratio calculator? or run the car up to speed in D2 and see if 60 mph at that engine rpm is a no-go. do you have a tach? if no maybe get a cheap POS and wire it in temporary just for finding your rpm tolerance threshold? How Fast Do You Want To Go?  ------------- 72 GT Ute |
mrbpatsfan wrote:Posted By: peter.jenerette
Date Posted: 23-August-2025 at 2:29PM
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I went from 2.73 open to 3.50 traction lok, and it immediately put a grin on my face, but then I hit some highway that rev'd it a bit more than I liked, so, I ended up swapping in a 4R70W to get OD which gave me the best of both worlds. ------------- Pete Jenerette 1972 Gran Torino (H-Code - 4R70W) 2022 F250 XLT 7.3 2003 Thunderbird |
Posted By: MARTY
Date Posted: 17-October-2025 at 6:07AM
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Good afternoon. Just got a 68 gt with the 428 but need to change the rears to something usable.350 or 325 How do I determine whether the axle is 28 or 31 spline. Everything on my build sheet leads me to 31 but I would like to make sure. Thank you ------------- M FLEMMING |
Posted By: californiajohnny
Date Posted: 20-October-2025 at 9:12AM
4 bolts... pull an axle out and count the spines .... ------------- JOHN 74 GRAN TORINO S&H CLONE 74 VETTE CUSTOM 90 S10 BLAZER 4X4 LIFTED 77 CELICA CUSTOM 75 V8 MONZA SUPERCHARGED 79 COURIER VERT. SLAMMED 75 VEGA V6 5 SPD 70 CHEV C10 P/U 68 MUSTANG FB CONVERSION |
Posted By: aquartlow
Date Posted: 20-October-2025 at 10:58AM
Totally agree with CJ, the most accurate way for a person to know the "actual" axle spline count....unless of course you can't count and/or remember  .------------- www.supermotors.net/22468 Yeah, It's a Hybrid...It burns gas and tires. No matter how good she looks, somebody, somewhere, is tired of her sh*t. Beauty is skin deep, ugliness goes clear to the bone. |