The ECM uses the sensor output for the speedometer, but also for the transmission and mixture controls. If it was a sensor, the trans would shift into O/D at 20 mph. It sounds more like the decoder chip or the circuit board at the back of the cluster that changes the pulse signals to a voltage signal that runs the speedo. Plug in a code reader that has live data capability and take it for a drive. Verify that the live data reads the proper mph's compared to the dash. If they are different, it's the C/B or decoder.
In rare cases, yes. The PCM has a common ground for the heated O2 sensor, Trans input sensor and a few other components. If there is an internal short in the heater system, it is possible it is grounding battery voltage through the O2's high/low signal circuit, scrambling the speed circuit signal.