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Discussion Starter · #1 ·
:confused0062: There is a 8.5" 10 bolt under a dead 75 Elky which I wanted but I needed to find out the gear ratio before laying out some green. The axles were covered with oily dirt crap so it was not possible to harvest some helpful numbers. But I knew I could find the ratio thanks to a helpful tip that appears on several sites: Turn the driveshaft and count the revs needed to turn one of the wheels one rev. The ratio can be approximated real close. That's what I did. Here are my results: to get one turn of the wheel, the driveshaft was rotated 1.33 turns. I couldn't believe it so I did it again.. same result. Then I rotated the driveshaft exactly one turn. The wheel rotated approximately 3/4 of a turn.
OK...so I feel stupid. :stupid: What went wrong? BTW, I was rotating the driveshaft by hand just in front of the rear universal. I had a helper (the anxious seller) watching the mark on the wheel as I turned the driveshaft.
Kep
 

· RIP: 10-27-2018
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3,223 Posts
:confused0062: There is a 8.5" 10 bolt under a dead 75 Elky which I wanted but I needed to find out the gear ratio before laying out some green. The axles were covered with oily dirt crap so it was not possible to harvest some helpful numbers. But I knew I could find the ratio thanks to a helpful tip that appears on several sites: Turn the driveshaft and count the revs needed to turn one of the wheels one rev. The ratio can be approximated real close. That's what I did. Here are my results: to get one turn of the wheel, the driveshaft was rotated 1.33 turns. I couldn't believe it so I did it again.. same result. Then I rotated the driveshaft exactly one turn. The wheel rotated approximately 3/4 of a turn.
OK...so I feel stupid. :stupid: What went wrong? BTW, I was rotating the driveshaft by hand just in front of the rear universal. I had a helper (the anxious seller) watching the mark on the wheel as I turned the driveshaft.
Kep
If you have one wheel on the ground you have to make two rotations so its somewhere in the range of 2.66 to 1
Gary
 

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Back in the day I junked a lot of those cars common ratios were 2.41, 2.56, and 2.73. I have never had any sucess with the spining the driveshaft method. Do not feel bad because I get results similar to yours.
All GM ring gears of that era have numbers stamped on them next to part # like 41 15 which is a 2.7333333, pull the cover & look. Otherwise the pass side tube has numbers & letters about the midpoint, Get those & I will look it up in Hollander.
 

· Trifive Automotive Electrical Wiring Expert
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As Gary said, you HAVE to turn ONE wheel 2 revolutions to get the driveshaft ratio because of the spider gears, if it's a non-posi. If it's a posi you can't turn one wheel, so you turn both wheels only one revolution, because the spiders are locked up on a posi.
 

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Discussion Starter · #7 ·
Got it! Thanks to Mitch, Gary, Tony, USA1, and Don for replies! Funny how I missed the need to make sure the wheel rotated two times. Oh well, it's a learning process. :)
For future readers, the rear end had both wheels off the ground, tranny was in neutral, and was determined to be a non-posi (when one wheel was rotated, the other rotated but in the reverse direction).
To USA1 - thanks for the Hollander offer but at this point I am not going back to this particular car. Now I know where to look for the numbers! Still looking for a 12 bolt with a lower ratio; > 3.00 to 1, and hoping to find a posi.
 
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