rear end of 66 c10

[reaper0001]

i have a 1966 chevy c 10 with a 327 powerglide and unknown rear end
the numbers on the case are

gm10
n
3856525
h285

i and currious as to what ratio these gears have so i know what to swap them out for

the reason for the swap is that my truck will do about 65 at 4000 in second

 

[Lakeroadster]

My '65 had "HA" stamped on the rear axle tube. I verified it was a 3.73: 1 gear set by pulling the rear cover, which is the best way to find out what's in it today. The truck is 47 years old, it may not have the original gear set.

Or ..... http://www.youtube.com/watch?v=2y9iqvO7RaU

I found this on another website... can't vouch for the validity:

http://sixty7ss.proboards.com/index.cgi?action=display&board=suspension2&thread=132

REAR AXLE IDENTIFICATION:

The production code number contains prefix letters and production date numbers. Axle identification is stamped on the differential carrier along with the date of manufacture.

Example: HB 0612 HB = 3.07:1 ratio, O6 = month built (June), 12 = day built(12th)

If your rearend came out from under a 66 C10, that tells me its a 1/2 ton truck and there are only three factory gear ratios for a 1/2 ton pickup, 3.07:1, 3.73:1 and 4.11:1. Here are the codes for each one.

JQ-4.11:1 HF-3.73:1 These are the only codes my book uses for 66 rearends
HA-3.73:1 HG-3.73:1
HB-3.07:1 HH-4.11:1
HC-3.73:1 HQ-3.73:1
HD-4.11:1​

Hope this helps!