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Originally Posted by buymeabmwm3
O/D is 4th gear on most auto trannies. try driving along at 50mph, then hit the O/D button to turn off O/D and you'll hear the car shift into 3rd gear. in most situations theres no need to turn off O/D, only if going down a mountain and you want to keep yourr car in a lower gear so you dont have to ride the brakes
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This is the "old-style" of O/D.
The true O/D function of an automatic changes the final drive gear on the output shaft on the transmission. So basically if you have a 4 speed auto, it's still a 4 speed with the O/D on or off. The difference is about 5-10% taller gears (all of them) with O/D on for increased fuel economy.
In the situation you described above, it's not shifting into 3rd per se, it's hitting a lower geared 4th.