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Change rear end, or change out driven gear?

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Old 04-08-2018, 05:18 PM
  #1  
angels_unawares
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Default Change rear end, or change out driven gear?

When I bought my '65 coupe, it had been used by a teenager to race. The original rear end had been replaced with a 9" rear end, torque converter changed, which required specially weighted flywheel. The speedometer didn't work, much less read accurately. I have replaced the coil springs which had been cut to lower it, the front shocks, all the suspension and steering componenets. Recently, I started on the back, with new leaf springs, shackles, and shocks. Now, what to do about the speedometer problem? Should I keep the 9" rear end, and change the driven gear? Or put the original rear end back on her? I was lucky to get the original rear end when I bought her. Both rear ends are rusty, and neither have an ID plate of any kind, so I have no way of knowing the gear ratio to calculate which driven gear to buy, if I keep the 9" rear end. Is there a set of common ratios for each size rear end, or do I have to open up the differential and count the teeth? I have tried asking these questions at the parts stores and of my male friends who do some shade tree wrench turning, and they look at me like a middle aged, 5 ft tall woman asking these questions is crazy. Crazy I may be, but I need some answers so I can do something! I need to do one or the other.
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Old 04-08-2018, 06:45 PM
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Gun Jam
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You could guestimate the ratio by rotating the drive shaft and watching the rear wheel. If it takes about 4 turns of the DS to one turn of the rear wheel its probably a 4:11 if is just shy of 4 turns probably a 3:80 if its about 3.5 turns of the DS to 1 turn of the rear wheel a 3:50 ratio

Im pretty sure those 9" diffs have no rear cover. like the 8" the guts extract from the front in a single heavy *** unit and its too much work to do to count teeth.

If its got a 9" now I would keep the 9"

If the speedo works you could also compare the speed vs GPS speed and select a driven gear using that method as well.
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Old 04-08-2018, 06:46 PM
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Originally Posted by angels_unawares View Post
When I bought my '65 coupe, it had been used by a teenager to race. The original rear end had been replaced with a 9" rear end, torque converter changed, which required specially weighted flywheel. The speedometer didn't work, much less read accurately. I have replaced the coil springs which had been cut to lower it, the front shocks, all the suspension and steering componenets. Recently, I started on the back, with new leaf springs, shackles, and shocks. Now, what to do about the speedometer problem? Should I keep the 9" rear end, and change the driven gear? Or put the original rear end back on her? I was lucky to get the original rear end when I bought her. Both rear ends are rusty, and neither have an ID plate of any kind, so I have no way of knowing the gear ratio to calculate which driven gear to buy, if I keep the 9" rear end. Is there a set of common ratios for each size rear end, or do I have to open up the differential and count the teeth? I have tried asking these questions at the parts stores and of my male friends who do some shade tree wrench turning, and they look at me like a middle aged, 5 ft tall woman asking these questions is crazy. Crazy I may be, but I need some answers so I can do something! I need to do one or the other.
By all means, keep the 9" rear end! Even if it needs some repairs.

First we need to determine whether it is a "locking" type rear end. Jack up one rear corner till the wheel clears the ground, transmission NEUTRAL, not in gear (if manual) or Park, if automatic transmission. Grasp the off-ground tire and attempt to rotate it. If it turns fairly easily, mark the rear end of the driveshaft with a black marker line. Start with a reference point, like that mark facing directly downwards, chalk mark the tire on its side facing the ground. Now have someone turn the driveshaft from under the car, noting exactly where it's mark was to begin with, while he/she counts the number of turns and the last partial turn of the driveshaft while you watch the tire mark. When the tire mark is again facing the ground yell "STOP", and find out how many turns the driveshaft made. If it is, say, 7 turns, divide the number by 2, and the result will be the gear ratio of the rear end, 3.50.

If the tire will not turn easily, or at all, you have a locking differential, and both wheels must be off the ground. Repeat the counting process, but don't divide driveshaft turns by 2.

Counting gear teeth on a 9" is rather difficult, as the design has no removable cover plate to access the ring gear. Follow this OK? If you get the gear ratio, we'll go on to the speedometer gears. imp
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Old 04-08-2018, 06:49 PM
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Oh yeah good point on the open VS LSD/Torsen diff!

I would just jack the car up from the bottom of the diff housing and lower it on jack stands if both wheels turn in the same direction when you rotate the DS its not an open diff. it has a clutch pack or / locked up or a gear driven torsen system

Last edited by Gun Jam; 04-08-2018 at 06:52 PM.
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Old 04-08-2018, 06:52 PM
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Originally Posted by Gun Jam View Post
Im pretty sure those 9" diffs have no rear cover. like the 8" the guts extract from the front in a single heavy *** unit and its too much work to do to count teeth.

If its got a 9" now I would keep the 9"
You got yours in one minute before mine! We think alike, but I'm too technical! I've been setting up 9-inchers since they came out in 1957! imp
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Old 04-08-2018, 06:54 PM
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Thats a solid skill to have...last I checked that 7.25 I rebuilt was still living good!
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Old 04-09-2018, 05:05 AM
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You can buy the right gear for the speedometer to correct the speed regardless of what rear end gear you have. The 9" rear is far superior to the 8", so I would keep it if it is good. As far as the gear ratio, it all depends on what you have for HP and how you are going to drive it. What is this car going to be, and how do you want to drive it? Daily driver, car shows, Sunday driver, New England Dragway for the 1/4, do you care about MPG?
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Old 04-09-2018, 09:18 AM
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I would say that 9" is probably no good, so remove it and send it to me. I will send you a perfectly good 8" with a removable back so you can count the teeth.

Only jokin' with ya. By all means do what the guys said, that 9" is hard to find.
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Old 04-09-2018, 02:15 PM
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angels_unawares
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Thanks all for the help. As soon as I get the new leaf springs in, I'll do the drive shaft/wheel turn. It seems that I remember the previous owner telling me that the diff ratio was 3.55, but I'll check it out. I know that when the speedometer reads 65, I'm only actually going about 40. From all the calculators online, it looks like I might need a 23 tooth gear, but I plan on doing the wheel turn, and checking the speedometer against a GPS, as well as checking the teeth on the driven gear. I replaced the speedometer and cable the last time I did any work on her, so at least I know they work.
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Old 04-09-2018, 02:18 PM
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angels_unawares
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The car will probably be used for something in between a daily driver and a Sunday ride. As hot as it gets in Texas, I doubt I will use her much when it's over 100 degrees, since she doesn't have an a/c.
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