Overdrive is an extra gear either in the transmission or between the transmission and the differential. In most of our older transmissions the top gear was a 1:1 ratio. For every turn of the input shaft the output shaft rotated exactly once. Lower gears like 1st and 2nd in a torqueflight used more turns on the input shaft to turn the output shaft once, allowing the engine to get the car rolling.

In overdrive, the output shaft spins more often than the input shaft usually a ratio like .80:1 or .70:1 allowing less engine rpm to achieve the same speed to the driveshaft.

The later A833 four speeds had an overdrive 4rth gear. If you some an older car or truck with a push button to activate the overdrive it may have had an external gear like a gear vendors unit that installs between the transmission and driveshaft.

The problem is, under load like towing, the engine actually has to work harder in overdrive. So most modern transmissions have a button that locks out the overdrive function that the truck would normally automatically switch into, rather than turning it on.