L.A.B. said:
If the cable inner projection is long enough to push the speedo drive spindle upwards into the speedo housing, as the spindle rotates the speedo spindle and housing will wear rapidly.
I think as tpeever said that 7/16" is correct, but you can check this by measuring both parts, how much the inner cable projects from the end of the outer ferrule (not the threaded sleeve) and the depth of the speedo spindle drive recess that it fits into, provided the inner cable length is less than the depth of the recess by at least 1/16" then it should be OK.