There is a difference between the output speed and input speed, which is known as the gear ratio. Generally, these speed sensors on both the input and output sides of the transmission are used by the ECM (Engine Control Module) to compare and determine if the transmission ratio is correct.
In case the Engine Control Module detects that the transmission ratio is not within the specified range, the Check Engine light will turn on the dash along with the DTC P0735 OBDII code.