8 3/4 rear end question

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omotz

omotz

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How do you tell what the gear ratio is. ID numbers? Spin the wheel and observe yoke rotations? My 63 is a manual and the rear end came out of a car with an automatic. I’m guessing the rear end has highway gears in it. I jacked the car up yesterday spun the wheel. One turn and observed the yolk looks like it turned about one and a half turns. Thanks, Tony

IMG_1045.jpeg
 
Not sure when I spin one wheel the other wheel does not turn that answers your question
 
Rotations x2 for open
Rotations x1 for trac lock

1.5x2 =3

2.94 open my best guess.

If you jack up only one side and you can rotate the the jacked up tire you have an open (or severely warn out trac lock)diff.
 
So I jacked it up, so both wheels were off the ground. Dana67dart not understanding your post… the first part. Thanks
 
Dana67Dart said:
Rotations x2 for open
Rotations x1 for trac lock

1.5x2 =3

2.94 open my best guess
Click to expand...
With an open diff rotate the tire 2 revolutions and count the yoke revs.

If it's trac lock rotate the tire 1 time.

You got 1.5 yoke revolutions for 1 tire revolution so multiply it by 2 gets you 3. And 2.94 is the closest 8 3/4 gear to 3
 
So I jacked up passenger side and it spins so that confirms it’s open and it’s a little bit more than one and a half yoke revolutions per one tire revolution so would that be considered highway gears or lower gears? Reason I ask is because I think I want the old style 833 tranny with the lower first gear not an overdrive tranny.
 
Jack up one wheel, and I assume there is no drive shaft. If you can turn that one wheel, with the other on the ground, it is open. Now turn that wheel TWO exact turns and count the driveshaft. That is your ratio If you did the 1T accurately 2X 1.5 is about 3, so might be a 3.23.
 
Dana67Dart nailed it. He's exactly right.
Depending on how accurate your observation was, either 2.94s (very much highway gear) or 3.23 (a pretty good compromise ratio.)
Either are considered high gears (low numerically).
Low gears, meaning low car speed for engine revolutions would be 4..11, 4.56 etc (higher numerically).
 
2.94 is closer to 3 than 3.23.

3.23 would be more than 1.5 revs more like like 1.6 revs, and hard to make a good count with 1 tire rotation.

If you want to get a more accurate number, rotate the tire 10x, count yoke revs, multiply by 2, move the decimal point left one position.

14.7 yoke revs will be 2.94

16.1 yoke revs will be 3.23
 
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