speedometer Revolutions per mile?

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Anybody know the revolutions per mile for 1964 C10 speedometer? I am converting to a 700R4 trans and need to convert the electric speedo output to a mechanical one and need to know for getting the right drive/driven gears.

They say that many times the number is stamped on the face or back side of the speedo? I couldn't see anything on the face of mine. Anybody have one they could check? It sure would be helpful. Thanks.

PS- tried a search but came up empty handed