41 Ford F15a CMP

[Tony Wheeler]

Originally Posted by Bob Carriere

and 6:39 is what is usually stamped on the outside of the gear cluster

You've just reminded me Bob - the diff ratio on Fords is stamped on the outside of the housing, adjacent to the filler plug, so it should be a simple matter for Rob to confirm. Here's a few examples on my own Fords, and a stores tag I found wired to a Ford diff housing recently. Presumably a replacement axle assembly - rather important to choose the correct ratio!