Verify You Have A Good Common Ground
Make sure that your speedo sender and the speedometer have a good common ground. If in doubt, ground them together on a good ground location.

Verify You Have A Good Signal
You can verify your signal by monitoring the pulses that get counted during calibration:

• Enter calibration mode
• If you see pulses being counted on the ODO while not moving then you most likely have signal noise. This can be due to a bad sender or wiring.
• Start driving a measured mile at a steady speed and have someone monitor the pulses that are being counted on the ODO display.
• If your signal is good you will see pulses count incrementally (not erratic). The pulses will increment at a faster rate the faster you go, so if you drive a steady speed (i.e. 25mph) you should see the pulse count increment at a steady rate. If the pulses increment erratically then you do not have a good signal. Erratic pulse count while driving a steady speed during calibration indicates you most likely have a bad sending unit.