need help identifying a GM rear end

I'm working on a 37 chevy project that I bought about 50% done. I need to identify the rear end and gear ratio to make sure it will be geared right for the 4-speed I will use. It is a 10 bolt rear end. there are no numbers
on the axle tubes but the carrier has GM71 on the front passenger side and the numbers 3969340 NF on the front driver side. I tried to determine the gear ratio but I'm not sure I did it correctly. I marked the pinion and the tire. blocked one tire and turned the other one 10 times; the yoke only turned 13 times. seems to be very high geared but I'm probably not doing it correctly. I would appreciate any help.
thanks,
Tony
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Best way here is to take the cover off and it should have a stamping on the ring gear like 11-37, divide the 11 into the 37 for the ratio, and check the condition while you are in there.

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You need to get both tires off of the ground and see how many times you have to spin the yoke to turn both wheels one complete revolution.
Or take the cover off and look for stampings. Failing that, just count the teeth.
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What you will need to cross reference those numbers is: Go to a salvage yard that has Hollander manuals going back to 1955 (first year of the 10 bolt). That will tell you what year, make and model of GM 10 bolt you have. Knowing that is important for brakes, gears, axles, and oil seals.
The way your checking the gear ratio is a ball park measurement. The correct way is to have both rear wheels off the ground. Then mark the drivers side drum & brake backing plate. Then work the pinion yoke and oil seal. Turn the yoke slowly and count the number of revolutions the brake drum turns. Only turn the yoke one revolution.
10 Bolt Rear ends are still made even now. They come in different types, and sizes. The first year for them was 1955. So that's almost 50 years of rear end codes to check. Except a 3969 should be a 55 to 1988 number. Charles
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Unless this is some bizzare rear end, isn't that process backwards? Turning the yoke exactly one revolution will turn the axles less than one. You'd have to turn the wheels one turn, counting the revolutions of the drive yoke (as previously stated).
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Yup. I made an oopps there. Thanks for catching it. Charles
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