Over drive is basically a gear one higher than the transmission says it has. Why not just say it is a 5 speed instead of calling it a 4 speed with overdrive you may ask. The reason is that the over drive gear has a higher gear ratio than dose a 5th gear. The reason for this is fuel economy. With an over drive the car dose not have enough torque to accelerator very quickly but it allows the engine to run at a lower RPM and using less fuel when highway driving. Because of the lack of torque you don’t want to use the overdrive if pulling a load or you can sauce a lot of damage to your transmission. Cars and trucks designed for towing don’t have an overdrive.