I've been searching the posts for differential ID numbers to no avail. I have a 79 auto with the differential ID number AO m 4 11 69W. The "m" is lower case and is stamped between the O and 4 near the top of the characters. I have no idea what this "m" is nor can I decipher the numbers. Maybe the diff was changed out by a previous owner but don't know. Anybody care to take a guess. One last thing, the car doesn't pull like it has a 3.70 rear more like a 3.08. I'm being lazy and don't want to jack the car up one more time today otherwise I'd do the spin the axle and count the revolution trick.

The only code my rsto book lists for a '79 are
OM 3.36:1
OH 3.55:1
OJ 3.70:1
I saw no AO for any C# model year.

Hello, what you have is a 1969 rear end, which originally came with 3:70-1 Posi gears,equipped with a smallblock.If you want to see for yourself, check the N.C.R.S.pocket guide, and it's listed there.Someone probably replaced yours with an off the shelf rebuilt one.This is very common. Bob

That's what I suspected. The code may be for a 3.70 rear but at 70mph the tach is turning around 2800 rpm. Could be the tach is not working correctly but the rear sure doesn't pull like a 3.70. I'll jack the car up sometime this week and count the turns on the axle. Thanks for the replies.

assuming your top gear is a 1:1 ratio.. ie th350,400, or 4 spd

then at 70 mph with 3.08 you would be at 2684 rpm

at 70 with 3.36 you would be at 2928 rpm

I thank you once again. I'll take a close look at the tach the next time I take a drive. I bet it's a 3.08. What I think I saw (senior moment) on the tach was around 2700. The 3.08 would make sense.