Speedometer cable weirdness

If you can completely remove the inner drive cable, the last inch or so of the end that goes toward the transmission "should" be square. The end at the instrument cluster should have a swaged fitting on it with a raised key way, or sometimes no fitting, but square, like the other end. I just went and looked at my 65 Barracuda, that I'm restoring...the cable is square on the instrument cluster end. Your cable may be broke somewhere in the middle, but touching together enough to allow the speedometer to work at a lower speed, but separates at higher speeds. It also maybe the drive gear in the transmission may be worn to the extent that it works a lower speeds but not under the stress of higher speeds. Some of the drive gears have a metal insert in the nylon gear to accommodate the square end of the cable. It may be slipping in the nylon gear. Just an idea...I'm sure someone will add to it for more help. Let us know what you do to fix it.
Norm