Ford Repair: What is overdrive, extra gear, highway speeds

Question
my friend thinks that overdrive gives the engine extra power to get up a hill so it is less stress on the motor, i have been told that it locks the gearbox. what is your opinion?

Answer
Daniel-

  Overdrive is really just an extra gear for freeway cruising.  It allows the engine to turn at lower rpm at faster highway speeds.  If anything, it would actually hurt you going up a hill since the gear ratio is lower and the engine will have to work harder to make it up the hill.  Hope this helps.

Steve