On most modern transmissions you dont have to do anything other than not lock the overdrive feature out . It automatically selects a higher gear ratio thereby decreasing motor rpm for distance driven thus giving higher gas milage than can be obtained at a lower gear ratio. This takes place when you travel on level surface or where you have sufficient momentum to overcome forward resistance. When you increase workload such as climbing a grade the vehicle will then downshift to a lower gear ratio to maintain speed.
