A default value of transmission input speed in a motor vehicle powertrain
having an automatic-shift multi-gear ratio power transmission is
continuously updated based on a measured value of the input speed and
other reliable speed data including transmission output speed and engine
speed. The default value is substituted for the measured input speed when
a malfunction of the input speed sensor is detected, and transitions back
to the measured input speed when the input speed sensor malfunction is no
longer present.