Simplest way to think of it is that you have an automatic transmission, and it will automatically select the right gears for you. Just put the car in Drive and go. Do not turn overdrive off. Your four speed transmission is acutally like a five speed. Automatic Transaxles, 1st, 2nd, 3rd, 4 th. Trans, Gear, Gear, Gear, Gear. Model, Ratio, Ratio, Ratio, Ratio. A140E, 2.810, 1.549, 1.000, 0.706 See the last gear ratio is less than "one" so technically it is "overdriven". The reason I say it operates like a 5 speed is that you also have torque converter lock up which links the flywheel to the transmission input shaft like a manual transmission with the clutch engaged. If you're climbing a hill in overdrive and observe the tachometer, first the torque converter lockup releases and the tach needle moves slightly, (almost like going from fifth gear to fourth), the if the hill steepens or you press the gas pedal further, at speeds under apx.
95 mph., the transmission will down shift into third gear.