What is the Final Drive?
The Final Drive is the last gearing used while transmitting the engine power to the wheels. It has two purposes. Firstly, it turns the power flow at the right angle from the propeller shaft to the rear axle. Secondly, it also provides a mechanical advantage (leverage) from the propeller shaft to the rear axle. The Final drive contains a pair of spiral-bevel gears comprising a pinion that connects to the propeller shaft and a ring gear that connects to a flange on the differential case.
The pinion gets the drive from the engine while the crown-wheel is attached to the wheels and rotates them. Usually, there are 3 to 4 times more teeth on the ring gear than the pinion. Thus, it provides the final speed reduction between 3:1 to 4:1. The drive pinion is an integral part of the pinion shaft. Its spline-end attaches to the end of the propeller shaft or the rear universal joint.
What is the Final Drive Ratio?
The final-drive ratio is the extent to which the rotating speed of the driveshaft (output) from the gearbox finally reduces before it reaches the driven wheels. If you alter this ratio, it also alters every gear's effective ratio, thereby affecting the vehicle performance and economy. Hence, the manufacturers term it as the final drive ratio. Basically, it is the final ratio of the gear-train.
Basically, it is the ratio of the gear's angular velocity that gives the input to output ratio. You can easily calculate the final drive ratio directly from the number of teeth on the crown-wheel and pinion. The manufacturers define the final drive ratio and mechanical ratio in such a way that it results in a number that forms an ideal association.
How does it work in a vehicle?
Let's refer to the diagram above. E.g., there are 8 teeth on the pinion and 30 teeth on the crown-wheel, as shown in the diagram. To rotate the crown-wheel (& thereby the wheels) by one rotation, the pinion needs to rotate 3.75 times more. Let's assume that the vehicle is traveling in the 4th gear whose gear-ratio is 1:1, which means the gearbox input is equal to its output.
In the 4th gear, to rotate the wheels by one rotation, the engine needs to rotate the pinion 3.75 times more. This means the engine itself needs to rotate 3.75 rotations in order to rotate the wheels by one rotation. Because