What is O/D (over drive) for? When would I want it on? When would I want it off? Why is it called that?
by 
Max Power
ANSWERS: 2
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You should always keep your car in OD, over drive lets the transmission and motor rev at higher rpm, where as regular D (drive) is good for lower gear operations, it's not really necessary.
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What overdrive really means is rpm greater than engine rpm. in overdrive the transmission rpm can be made greater than the engine rpm. This is achieved by using a gear train in which the driven gear wheel is smaller than the driving gear wheel. This is used for cruising and is more fuel- economical. For ex in manual transmission usually the 5th and 6th gear (if provided) are overdrives (rpm is greater than engine rpm). Although I have yet to see an automotaic drive with overdrive option however I believe it should be used when you are cruising at high speeds. However if you want to accelerate or overtake quickly this is not a recommended mode.
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