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I didn't think to measure mine when I replaced it, but it is unique to 68's.
You can also buy a replacement inner cable to go in your jacket. That you cut to fit and crimp the tip on the cable. That's how I fixed my wore out tach cable and kept the original 68 tach cable jacket.
If I remember correctly the inner cables tips were just short of the cables fittings that screw into the tach and the 4 speed.
Hope this helps. :chevy