47-55.1 Rear Axle Identification/Ratio

[Livemeyer]

When I bought my truck ('51 3100 5-window) in 2016 all indications were that at some point someone moved over a bunch of parts from an early or mid-60's Impala. It came with an Impala steering wheel which was the biggest giveaway, plus the aluminum Powerglide transmission, and all wheels had been converted to 5-lug Chevy, 5x4.75. That, and the owner told me it once had a 327 in it.

Ever since, I've wondered about the rear axle. Either the entire axle was swapped out, or the Impala's 5x4.75 splines were slid into the original housing. My bet is that it is most likely the former. So I have tried to identify it, and what possible gear ratio it is. I have 18" rear wheels with 255-40-18 tires. I put a 327 into it along with the Powerglide. I took it on the freeway first time about 10 days ago. It runs about 2,000 rpm at 40mph and 3,000 rpm at 60mph, which sounds like the motor wants to implode.

I found a stamp on the rear of the housing, a "B" and a code, 3707306. There's a tag next to the GM stamp but it's unreadable. I might try again to see if I can get something from this tag. Running 3707306 on Google I get a bunch of car websites like Tri-Five Chevys, who claim that number is a mid-50's stamp, and the "B" indicates it was made in Buffalo. None of this really jives with idea that this would have come from a 64 Impala, but we're close. Also, I can't determine from any of this what the gear ratio is. Based on driving characteristics, I'd think this may be as low as a 3.70 ratio or as high as a 4.11 ratio, but most likely a 3.90 ratio.

Can anyone help me identify this axle? Can we at least say for certain this is not the factory original 51 3100 truck axle?

[Livemeyer]

According to Summit Racing, it looks like I'm likely running a 3.90 or 4.11 ratio rear, according to this chart on their website. My 18" wheel and tire combo are approximately 28 inches in total diameter, so I'm seeing 2808 and 2952 as my rpm ratios at 60mph.

[mongocanfly]

255/40-18 should be a 26in tall tire.....not 28 ...although tire sizes arnt always what they say they are
a 3.90 gear and 26 in tire and pg ratio of 1:1 would give you around 3024rpm at 60mph

i use this to determine rpms...
http://www.csgnetwork.com/multirpmcalc.html

and this for tire sizes
https://tiresize.com/chart/

that could possibly be a ratio tag on the diff bolt

[mongocanfly]

that 3707306 number keeps showing a 55-56-57 rear axle
should be a number beside the GM ...under that tag
remember..a lot can happen in over 60yrs
if you really cleaned up the 3rd member you may find some stamped numbers on it

[Livemeyer]

Originally Posted by mongocanfly

that 3707306 number keeps showing a 55-56-57 rear axle
should be a number beside the GM ...under that tag
remember..a lot can happen in over 60yrs
if you really cleaned up the 3rd member you may find some stamped numbers on it

I suppose it really could be a 55-57 axle. No clue on ratio though. But I will be doing more cleanup on the axle (I plan to paint it) so like you said maybe I'll find some more numbers there.

[Livemeyer]

Originally Posted by mongocanfly

255/40-18 should be a 26in tall tire.....not 28 ...although tire sizes arnt always what they say they are
a 3.90 gear and 26 in tire and pg ratio of 1:1 would give you around 3024rpm at 60mph

i use this to determine rpms...
http://www.csgnetwork.com/multirpmcalc.html

and this for tire sizes
https://tiresize.com/chart/

that could possibly be a ratio tag on the diff bolt

Yes you're right, 26... I want to get fatter meats on those 18's (not width, but tall) so 28 stuck in my head because I was researching what tire ratio gets me to 28 or taller. I'm considering going to 45 ratio, maybe 275/45/18, but could be 265 or higher, I just have to check for clearances before making a change.

[jwhotrod]

if it is a 55-57 chevy rearend be aware that those years are all single year bearing (axle) and back in the 80's I had one in a 34 chevy coupe. the bearings were difficult to find then. so I would find a newer axle if I were you. Also beware of 8"and9" ford axles. they are fusion welded at the axle/flange joint and weak, when that weld lets go bad things happen. I have a 8" rear in my 34 chevy (replaced the 57 rear) the left flange sheared of at 20 mph, the wheel and drum tore the left rear fender off the car ad bet up the body pretty badly. Since I heard of a 37 ford that had that happen at highway speed, car turned left and rolled. When I talked to the guys at Strange Engineering, he said yes they are weak, (all of them) and they will fail with NO WARNING at all. So know both of my cars, the 34 cpoupe and my 35 chevy sedan both have 8" rears with Strange axles in them. So good luck with your build.

[Livemeyer]

Originally Posted by jwhotrod

if it is a 55-57 chevy rearend be aware that those years are all single year bearing (axle) and back in the 80's I had one in a 34 chevy coupe. the bearings were difficult to find then. so I would find a newer axle if I were you. Also beware of 8"and9" ford axles. they are fusion welded at the axle/flange joint and weak, when that weld lets go bad things happen. I have a 8" rear in my 34 chevy (replaced the 57 rear) the left flange sheared of at 20 mph, the wheel and drum tore the left rear fender off the car ad bet up the body pretty badly. Since I heard of a 37 ford that had that happen at highway speed, car turned left and rolled. When I talked to the guys at Strange Engineering, he said yes they are weak, (all of them) and they will fail with NO WARNING at all. So know both of my cars, the 34 cpoupe and my 35 chevy sedan both have 8" rears with Strange axles in them. So good luck with your build.

Wow, that's some crazy good information to have. Sorta glad to know, sorta wish I didn't, you know what I mean? I eventually want to do IFS on the front end, but now that I know this I might want to go independent on the rear too.