Here is the numbers on my rear end housing 3725893( the last no# could be 8 or 9) There is another no# D167 and GM23. I turned the wheel one full rotation 360 degrees and the driveline turned 1 5/8(actually closer to 1 11/16 turns) Hope someone can figure out the rear end ratio. Thanks, Harold Pickett

Did you have the other wheel locked or on the ground? Or was it free to rotate?

The method I use is to have the tires on the ground and move the car one rotation of the rear wheel(s) and count the rotations of the drive shaft. 4 1/10 turns would be a 4.11, 3 and 3/4 turns would be a 3.70. and 3.5 turns would be a 3.55. The ovedrive models had a 4.11, sticks a 3.70 and PG had a 3.55. ('57 had a 3.36)
The long number is a casting number,
The letters that tell the tale would be AA, AB, AC, AL, AM, BA, BB, and BC. with some date numbers following.
These are not raised numbers but rather stamped into the flat surface facing the front of the car on the differential center carrier, right side, in the 9:00 o'clock position, mid-way between the banjo housing to carrier bolts. The location is pictured in the shop manual --page 2.

Harold,

If you have an open rear axle (non positraction). Raise one rear wheel only off the ground. One person slowly turns the raised wheel 2 complete revolutions while a second individual counts the prop shaft turns. Then use the same description Gene recited above. 3.7 turns = 3.70:1.

Better yet, rotate the raised wheel 10 full revolutions while counting the prop shaft turns. Then double the counted number to obtain the axle ratio. Or reduce the error further by rotating the raised wheel 20 full revolutions and read the prop shaft turns directly to obtain the ratio.

Gene's method works because, when on the ground, both rear wheels rotate the same. With an open axle and both rear wheels off the ground, if you rotate one rear wheel without holding the oppoite rear wheel stationary anything can happen and the counted number means zilch because of the differential. The opposite wheel might turn the opposite direction, only turns slightly as fast as the turned wheel, or what ever.

Positraction axles are easier to determine. Raise both rear wheels, turn one wheel 1 full revolution while counting the prop shaft revolutions (the opposite wheel will follow). Or for better accuracy, turn one rear wheel 10 full turns and count the prop shaft turns directly.

Chevgene: here is what I found about the rear end info--- On the right side of the rear end is the letters and numbers which are "AB 4-22" I used your method of having the tires on the ground and rotating the wheel one revolution and I got a driveshaft rotation of 3 3/8 revolutions( Is this a 3:36 ratio?) This is a 57 chev rear end that the owner of Dutchman rearends in Portland shortened both axles by 1 1/2" so you could use wider tires. Its been several years since I bought the rear end from them that came out of there 57 that was replaced with a 9" ford rear end. The rear end was rebuilt when I purchased it. Thanks for the info----I want to thank you for all the help you gave me on the restoration of a 1940 pickup that I restored for a gentleman in Moro Bay, Ca. He wanted the truck restored back to 1940 specks, it was your infomation when I needed it that brought the restoration to a final completion, again, thank you--Harold Pickett

Chipper: I had One wheel off the ground. By using this method I got one revolution of the tire which gave me 1 11/16 revolutions of the driveline. According to Chevgenes method seams to work and if he is right it is a 3:36 ratio from a 57 chev rear end by keeping both tires on the ground and turn the wheel one rotation and count the revolutions of the driveline which came out ot be 3 3/8 revolutions which maybe a 3:36 ratio. Thanks for the reply, Harold Pickett

Differentials act differently when you have both rear wheels off the ground and turn one wheel when the car is in gear or in park. I learned that as a 12 year old with a model A Ford. I tore the differential down to see what was wrong with it when I turned one wheel and the one on the other side TURNED the opposite direction. My Dad cane out to see what my problem was....He gave me a short lesson on how differentials work. I still remember that lesson.... Maybe acting differently is what a differential does best!

If you only move one wheel and the other is stationary then the rotation measured will be 1/2 that when both wheels are on the ground and moved forward or back. So your 1 11/16 revolution becomes that times 2 or a 3.36 ratio. The number of turns when both wheels are on the ground will be the ratio as you have already discovered.

I wouldn't trust the above as the "slop" in the pinion gears and ring gear and pinion would make the difference between the 3.36 and 3.55 ( or 3.55 and 3.70) difficult to measure. The 3.36 was not used in 1955 or 1956 and was a 1957 automatic transmission ratio.
AB would be a 3.55 from a 1955-56. The numbers are the day/month. Year is not stamped so could be either 1955 or 1956.
The complete carriers on these cars was often switched around when the gears/bearings got noisey but very seldom were different ratios installed in the carrier as they were a more difficult rear end to set-up.
After you get the ratio determined the "computer" would be set for the original tire size for engine speeds. If you are running something other than a 6.70 X 15 tire the RPM's would not match the gear ratio.