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?