Hey, recognize that schema ;-) Remember to put a voltage divider on the Tx.
The voltage you measure seems to be about the max of the range, as indicated in motor.ino line 60. That would happen when R6 is not connected to ground. In that case, the input would be clipped by the protection diodes inside the arduino. It would be clipped against 5V, and with the diode voltage drop, you would measure 5.6V over it. So, I'd give that R6 a good scrutany. BTW this would also happen if you swapped R3 and R6 accidentally.
The voltage you measure seems to be about the max of the range, as indicated in motor.ino line 60. That would happen when R6 is not connected to ground. In that case, the input would be clipped by the protection diodes inside the arduino. It would be clipped against 5V, and with the diode voltage drop, you would measure 5.6V over it. So, I'd give that R6 a good scrutany. BTW this would also happen if you swapped R3 and R6 accidentally.