The purpose of the ultimate drive gear assembly is to provide the final stage of gear reduction to decrease RPM and increase rotational torque. Typical final drive ratios can be between 3:1 and 4.5:1. It is because of this that the wheels by no means spin as fast as the engine (in almost all applications) even though the transmission is in an overdrive gear. The final drive assembly is connected to the differential. In FWD (front-wheel drive) applications, the final drive and differential assembly can be found inside the transmission/transaxle case. In an average RWD (rear-wheel drive) program with the engine and tranny mounted in the front, the ultimate drive and differential assembly sit in the rear of the automobile and receive rotational torque from the transmitting through a drive shaft. In RWD applications the final drive assembly receives input at a 90° angle to the drive tires. The final drive assembly must take into account this to drive the rear wheels. The purpose of the differential is to allow one input to drive 2 wheels as well as allow those driven tires to rotate at different speeds as a vehicle goes around a corner.
A RWD final drive sits in the trunk of the automobile, between the two back wheels. It is located in the housing which also may also enclose two axle shafts. Rotational torque is used in the final drive through a drive shaft that runs between your transmission and the ultimate drive. The ultimate drive gears will consist of a pinion gear and a ring gear. The pinion equipment receives the rotational torque from the drive shaft and uses it to rotate the ring gear. The pinion equipment is much smaller and has a lower tooth count than the large ring equipment. Thus giving the driveline it’s last drive ratio.The driveshaft provides rotational torque at a 90º angle to the direction that the wheels must rotate. The final drive makes up for this with the way the pinion gear drives the ring gear within the housing. When setting up or establishing a final drive, the way the pinion gear contacts the ring gear must be considered. Ideally the tooth get in touch with should happen in the precise centre of the band gears the teeth, at moderate to complete load. (The gears press from eachother as load is applied.) Many final drives are of a hypoid style, which means that the pinion gear sits below the centreline of the band gear. This allows manufacturers to lower the body of the automobile (because the drive shaft sits lower) to improve aerodynamics and lower the vehicles centre of gravity. Hypoid pinion equipment tooth are curved which in turn causes a sliding actions as the pinion equipment drives the ring equipment. It also causes multiple pinion equipment teeth to be in contact with the ring gears teeth making the connection stronger and quieter. The band equipment drives the differential, which drives the axles or axle shafts which are connected to the trunk wheels. (Differential operation will be described in the differential portion of this article) Many final drives house the axle shafts, others make use of CV shafts like a FWD driveline. Since a RWD final drive is external from the transmission, it requires its oil for lubrication. That is typically plain equipment essential oil but many hypoid or LSD final drives need a special kind of fluid. Make reference to the services manual for viscosity and additional special requirements.

Note: If you are likely to change your rear diff fluid yourself, (or you intend on starting the diff up for services) before you let the fluid out, make sure the fill port could be opened. Nothing worse than letting liquid out and then having no way of getting new fluid back in.
FWD final drives are very simple compared to RWD set-ups. Virtually all FWD engines are transverse mounted, which means that rotational torque is created parallel to the path that the tires must rotate. You don’t have to modify/pivot the path of rotation in the final drive. The ultimate drive pinion gear will sit on the end of the result shaft. (multiple result shafts and pinion gears are feasible) The pinion gear(s) will mesh with the final drive ring gear. In almost all situations the pinion and ring gear will have helical cut the teeth just like the rest of the transmission/transaxle. The pinion equipment will be smaller and have a lower tooth count compared to the ring equipment. This produces the ultimate drive ratio. The ring gear will drive the differential. (Differential operation will be explained in the differential section of this article) Rotational torque is sent to the front tires through CV shafts. (CV shafts are generally known as axles)
An open differential is the most common type of differential found in passenger vehicles today. It is definitely a simple (cheap) style that uses 4 gears (occasionally 6), that are known as spider gears, to operate a vehicle the axle shafts but also allow them to rotate at different speeds if necessary. “Spider gears” can be a slang term that is commonly used to describe all the differential gears. There are two different types of spider gears, the differential pinion gears and the axle side gears. The differential case (not casing) receives rotational torque through the band gear and uses it to drive the differential pin. The differential pinion gears trip on this pin and so are driven because of it. Rotational torpue is certainly then used in the axle side gears and out through the CV shafts/axle shafts to the tires. If the vehicle is travelling in a directly line, there is no differential actions and the differential pinion gears only will drive the axle side gears. If the automobile enters a turn, the external wheel must rotate quicker than the inside wheel. The differential pinion gears will start to rotate because they drive the axle side gears, allowing the outer wheel to speed up and the inside wheel to decelerate. This design is effective provided that both of the powered wheels have traction. If one wheel doesn’t have enough traction, rotational torque will observe the road of least resistance and the wheel with small traction will spin as the wheel with traction won’t rotate at all. Because the wheel with traction is not rotating, the automobile cannot move.
Limited-slip differentials limit the quantity of differential action allowed. If one wheel starts spinning excessively faster compared to the other (way more than durring normal cornering), an LSD will limit the swiftness difference. That is an advantage over a regular open differential style. If one drive wheel looses traction, the LSD action allows the wheel with traction to obtain rotational torque and allow the vehicle to move. There are several different designs currently in use today. Some work better than others based on the application.
Clutch style LSDs are based on a open up differential design. They have another clutch pack on each of the axle aspect gears or axle shafts inside the final drive casing. Clutch discs sit between the axle shafts’ splines and the differential case. Half of the discs are splined to the axle shaft and the others are splined to the differential case. Friction material is used to separate the clutch discs. Springs place strain on the axle side gears which put pressure on the clutch. If an axle shaft wants to spin faster or slower compared to the differential case, it must get over the clutch to do so. If one axle shaft tries to rotate faster than the differential case then your other will attempt to rotate slower. Both clutches will withstand this action. As the speed difference increases, it becomes harder to overcome the clutches. When the vehicle is making a good turn at low Final wheel drive quickness (parking), the clutches provide little resistance. When one drive wheel looses traction and all of the torque would go to that wheel, the clutches resistance becomes much more apparent and the wheel with traction will rotate at (near) the rate of the differential case. This kind of differential will likely need a special type of fluid or some kind of additive. If the fluid is not changed at the correct intervals, the clutches can become less effective. Resulting in little to no LSD action. Fluid change intervals differ between applications. There is certainly nothing wrong with this style, but remember that they are only as strong as an ordinary open differential.
Solid/spool differentials are mostly used in drag racing. Solid differentials, just like the name implies, are completely solid and will not allow any difference in drive wheel speed. The drive wheels at all times rotate at the same velocity, even in a turn. This is not a concern on a drag race vehicle as drag automobiles are generating in a directly line 99% of that time period. This can also be an advantage for vehicles that are becoming set-up for drifting. A welded differential is a normal open differential that has acquired the spider gears welded to create a solid differential. Solid differentials certainly are a good modification for vehicles created for track use. For street use, a LSD option will be advisable over a solid differential. Every change a vehicle takes will cause the axles to wind-up and tire slippage. This is most apparent when driving through a slow turn (parking). The result is accelerated tire use as well as premature axle failing. One big advantage of the solid differential over the other types is its strength. Since torque is applied right to each axle, there is no spider gears, which will be the weak spot of open differentials.