When do I use the overdrive?

you are on a ramp merging into a highway, but the ramp is too short for your car to get to the highway speed. while pressing the gas pedal you switch the over drive off, the transmission will downshift to third to give your car that extra kick to reach the desired speed. you press the O/D switch again and the transmission will shift back to 4th. "as soon as you press the O/D button an indicator will light up and you will feel the engine's higher noise and rpm" (DO NOT FORGET To Turn It Back On Though cruising on the highway in third strains the engine unnecessarily)

Overdrive is used to get better fuel mileage and save engine wear at highway speeds. It IS NOT meant to be used at speeds under 50 mph. To use it in the city commute places too much stress on the engine (lugging), clutch (you are riding it to keep the engine in the proper rev range), transmission (constantly shifting & using the clutch), brakes (oops, too high a gear for engine braking). When GM first brought our their "OD" automatic in the early '80s, we found that we kept them in 'Drive' (3rd), not 'OD' (4th), because they kept hunting for the right gear between 30-40 mph. It was very annoying to feel the tranny constantly shift up and down in normal city traffic. As it turned out, those early GM 'OD' trannies had to be replaced a lot under warranty until GM realized that 'OD' works better under a constant load, not city driving. So, use overdrive only on the highway, shift down for on & off-ramps & for passing. Remember, you have less engine power (& conversely, engine braking) available in overdrive because of the gearing, so use wisely. Regards, Jim