What is the primary purpose of an overdrive unit fitted in the transmission system of an automobile?

1. To increase output torque for heavy load towing and hill climbing.
2. To allow the output shaft to rotate faster than the input crankshaft (ratio < 1:1), reducing engine rpm and improving fuel economy at high cruise speeds.
3. To reverse the direction of the output shaft during deceleration.
4. To act as a secondary torque multiplier during vehicle launch.