speedometer broken

Where to start? I have a m21 4spd, 3.55:1 posi, 275 60R15 tires on A 67 big block 4k car. I calculated the required speedo Gear to be 17 teeth - purple gear. When I put it in and drove the car over 60, My speedometer spun out of control. It spun to 120 mph and made a god aweful high...