Confused on diff ratio, help please
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Senior User
Joined: Nov 2007
Posts: 203
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From: Denver, CO
Confused on diff ratio, help please
I am now getting confused on how to tell what ratio I have without tearing my diffy apart.
Many times I have read that all I need to do is lift the back end of my truck, mark the driveline with tape, mark the tire with tape and spin the tire around one time and count the number of times the driveline turns. Well I was curious so I did that. I rotated the tire one turn and my driveline went 1.5 times around.
So, does that mean I have 1.50 ish gear ratio?
Did I do something incorrect or what? I have a 1972 9" diffy from a "camper special" and a C6 trans behind a 429 in a '52 F1.
Boy I did not know that Ford put out a 1.50 gear ratio.
I need help.
Many times I have read that all I need to do is lift the back end of my truck, mark the driveline with tape, mark the tire with tape and spin the tire around one time and count the number of times the driveline turns. Well I was curious so I did that. I rotated the tire one turn and my driveline went 1.5 times around.
So, does that mean I have 1.50 ish gear ratio?
Did I do something incorrect or what? I have a 1972 9" diffy from a "camper special" and a C6 trans behind a 429 in a '52 F1.
Boy I did not know that Ford put out a 1.50 gear ratio.
I need help.
Super Moderator
Joined: Apr 2004
Posts: 34,517
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Double that number. You probably have a 3.0, 3.25 if you were off a bit.
No tag?
No 1.5 ratio. The highest ratio I've seen here is 2.47.
3.25s make sense for cramper special.
Got a working tach and accurate speedo?
No tag?
No 1.5 ratio. The highest ratio I've seen here is 2.47.
3.25s make sense for cramper special.
Got a working tach and accurate speedo?
Thread Starter
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Senior User
Joined: Nov 2007
Posts: 203
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From: Denver, CO
No Tag, got a good tach and a very accurate speedo.....it sounds correct if I double it to get 3.0. Just never heard of doubling before. Thanks.
Senior User
Joined: Jun 2014
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I highly doubt you have a 3:1 rear end. Do the test again. 3.55:1 is the common super tall ratio, with 3.42:1 being an even taller oddball. There are taller ratios like 2.73, but probably not in a truck.
The reason you spin the wheel twice is because the differential gears are "walking" to make up for the other tire on the ground. So turn the tire two full turns, count the turns at the driveshaft and equate to a common ratio. IE a tad more than 3.5 turns is probably 3.55.
The reason you spin the wheel twice is because the differential gears are "walking" to make up for the other tire on the ground. So turn the tire two full turns, count the turns at the driveshaft and equate to a common ratio. IE a tad more than 3.5 turns is probably 3.55.
Frmr Ford Trans Engr
Joined: Aug 1999
Posts: 24,738
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From: SE Florida
The accurate method is to only have one wheel off the ground. With two off the ground the other wheel may turn some and distort your measurement. Of course you can't do this if it is a limited slip differential.
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Lead Driver
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From: SoCal
If it's a Ford traction lock differential, the break-away torque should be around 40 lbs-ft, so you sort of can turn one wheel with the other on the ground, but it will take some effort. Otherwise, raise the whole rear end up and turn the wheel once; the limited slip clutches (or locker) SHOULD keep both sides connected while you turn.
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