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overdrive is the highest gear in the transmission. On most cars the automatic transmission has 3 speeds and Overdrive (forth speed). It allows the engine to have less rpm with higher speed in order to have better fuel efficiency. When you switch it on, you allow the transmission to shift into overdrive mode after the certain speed is reached (usually 30 - 40 mph depending on the load). When it's off, you limit transmission shifting by third speed.
In normal driving condition the overdrive should be always on.
You also may need to switch it off when you want to slowdown using the engine braking, for example, driving downhill. |
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