1971 F100 locking 9 3/8" rear axle

[tonyc]

Okay all of you F100 experts. I decoded the tag on my rear axle and it comes up a 3:50 ratio, a locking rear axle, 31 spline. The strange part is that the ring gear is not a 9" as I expected - but a 9 3/8"! It sure looks like a 9". Is this part of a tow package or what gives? According to the decoding chart on this website this appears to be quite rare (WFF-B). The truck is a Ranger XLT with everything on it but I don't even know if they had a tow package on these trucks. Any enlightenment from the masses? Thanks ahead of time.

[BobbyFord]

Same diff that I've got in my '72. Some Lincolns came with a 9 3/8" also, I got one out of a '70 Mark III with 2.82:1 ratio.
Facing the pinion yoke it should have a turned-down webbing fin at about 2 o'clock.

[averagef250]

I had one in a '70 F-100. I don't think they're as uncommon as the books say. Seen a couple 31 spline LS narrow truck 9 inchers lately. They're hard to get parts for, but a real strong setup.

I'm pretty sure Ford selected a rear axle for trucks based mostly on GVW, but engine size may have played a part as well.

I've owned considerably more 4x4's than 2wd's and the relationship between the VIN GVW and axle size is pretty clear.