It is a mechanical  comparator which uses gear ratio  to increase  the speed at lower rpm so that vehicle can cruise at decent speed at low engine rpm and low noise and vibration. 

In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.

Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1

So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.

Overdrive is the top gear in the automobiles whose gear ratio is lower to the drive shaft allowing less fuel consumed at the same speed when compared with the other gears and also overdrive alowes for long 🏃💨 without heating of the engine and the best part is the maximum fuel efficient gear is overdrive gear ratio.

Overdrive or OD is the highest gear in the transmission in an automatic car. It brings the RPM of the engine down at a given road speed to facilitate better speed and fuel efficiency. It also helps your car to provide the best performance in higher speed cruising. Overdrive helps when you drive at more than 50mph.

2 hours ago, malek1 said:

Overdrive or OD is the highest gear in the transmission in an automatic car. It reduces the RPM of the enginen at a given road speed to facilitate better speed and fuel efficiency. It also helps your car to provide the best performance in higher speed cruising. Overdrive helps when you drive at more than 50mph