The speedometer cable is square to fit into the drive gear and to fit into the speedometer head.
It runs inside a cable housing, that unscrew at the trans end and has a fitting a the speedometer end. On the trans end, the cable screws onto a housing or mount that fits into the trans and mounts the driven gear.
If you unscrew the housing at the trans or the dash end you can pull the whole cable out. If it seems too short, its broke. Some of these, especially older Mustangs, have a cruise control sensor in the middle of the speedo cable, so there are two pieces of cable and housing.