Ring-and-Pinion
Residing within the housing centersection or third member is the ring-and-pinion, also referred to as the gearset. These are the parts that redirect the rotational force coming longitudinally down the chassis from the engine via the driveshafts and send it laterally outwards to the tires. The pinion is attached to the driveshaft via a U-joint and pinion yoke or flange, and turns at the same speed as the driveshaft while supported by two or three pinion bearings. At the end of the pinion are gear teeth, and they mesh with the teeth on the ring gear. The ring gear turns at the same rotations per minute (rpm) as the tires, and the ratio of the number of gear teeth on the pinion to gear teeth on the ring gear determines your ring-and-pinion ratio. So if the pinion has eight teeth and the ring gear has 41 then the pinion must turn roughly 5.13 times (41 divided by 8) in order for the ring gear to turn just once, thus this gearset has a 5.13:1 ratio.
The torque that the engine makes in one rotation (after being multiplied by the transmission and transfer case) is multiplied by the ratio of the ring-and-pinion. Using 5.13: 1 as the example, the engine torque in one rotation of the driveshaft is applied to turning the tires just 1/5.13 turn or about 20 percent of a revolution. If the gearing were 4.10 it would have to turn 25 percent of the revolution. However, with equal size tires in equal transmission and transfer-case gears, the 5.13 ratio would also require the engine to spin approximately 25 percent faster per single rotation of the ring gear versus the 4.10 ratio gears. Thus when that revolution is put over time you can see why a 5.13 gear would need to turn 5,000 rotations per minute (rpm) to turn the tires at the same speed a 4.10 gear would be turning them at around 4,000 rpm.
If that has your head spinning, try following this bit: The higher the ratio, the "lower" the gear is said to be, such that a 5.13 gear is considered 'lower" than 4.10 due to the fact that it offers more torque multiplication, similar to how low range offers more torque multiplication than high range in a transfer case.
Reverse Rotation vs. Standard Rotation (also known as High Pinion vs. Low Pinion) If you are confused at this point you may want to skip over to one of our feature truck or trail ride stories for a bit and let the facts sink in a while. If you're coming back for more, we'll now touch on reverse rotation versus standard rotation axles. More common standard rotation ring-and-pinions have a helical-style cut that delivers maximum strength when located in the rear axle of a vehicle. This is because the teeth on the ring gear are cut with a "drive side" designed to take the loads better than the "coast side" or backside of the teeth, but it also results in a lower pinion design with the pinion below the centerline of the axletube. Though this axle ring-and-pinion can be used in the front of a vehicle, it actually makes for a less desired setup because the ring gear is now being turned on the weaker "coast side" of the gear. A reverse rotation gearset is cut in the opposite way, making it stronger when used in the front of the vehicle because it is now running on the drive side of the gear, plus it has the more desirable high-pinion design with the pinion engaging the ring gear above the axle centerline. However, each of these gearsets requires its own specific housing centersection, so you cannot put reverse-rotation gears in a low-pinion housing or vice versa. Also if you turn a low-pinion housing upside down it does not make it a high-pinion housing. In fact it will make the tires turn backwards. To sum it up, a reverse-rotation (high-pinion) axle is stronger in the front of a 4x4 than in the rear, a standard rotation (low-pinion) axle is stronger in the rear of a vehicle, and turning a low-pinion axle upside down does not make it a high-pinion.