Ford Escort overdrive?

My mom has a Ford taurus with the same set up. The two overdrives are the same thing i dont really know why but my dad says that in over drive at 55 mph the rpm's drop down to half for more torque it is for pulling if you did. the regular gives the normal rpm's and is not made for any pulling. it shouldnt waste any more gas.I cant answer if it would hurt to swithch between the two i have never done it and probably wont just don want to take the chance. hope this helps.