Overdrive?

Can someone tell me what overdrive is, when do you use it, on the highway towing or street rideing. What is better on gas, overdrive off or on

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Typical 4 speed overdrive automatic: Overdrive is just a gear ratio. Gears 1-2 are underdrive and multiply torque. Meaning the input is faster than the out put. Gear 3 is not an under drive or overdrive and does not change the torque going through it it's ratio is 1:1 meaning the input is the same as the output. Overdrive is the input speed is slower than the output. It reduces available torque. So towing is not a good idea as that the engine will be under more strain when in overdrive. Fuel mileage will be better with overdrive on in normal conditions.

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