Most of us have heard of a “rear differential” or a “front differential” but few know what it actually does. Basically, the differential takes power from the vehicle’s transmission and turns power traveling down the centerline of the vehicle and transfers it to the wheels. It also allows the wheels to turn at different speeds while power is being applied. The picture above shows the components that allow that to happen.

The reason the wheels need to rotate at different speeds is because in a turn, the outer wheel travels much farther than the inner wheel. Here is a diagram showing how that works.


The differential gears are lubricated by “differential gear oil” which is quite different from motor oil or transmission oil. It is typically a much thicker lubricant. The oil is transferred to the outer wheel bearings in many vehicles only by the splash effect of the spinning gear set. That’s why it is extremely important for the gear oil to be checked at every oil change. A low gear oil level can cause extremely expensive damage to the bearings and gear sets.

There is no filter to filter out metal from normal wear so most differentials have a magnet in them to catch those metal particles to reduce any accelerated wear due to tiny pieces of worn metal.

Differential fluid does break down over time so servicing the differential is critical to avoid expensive breakdowns and repairs. Most differential fluid exchange intervals are at or over 60,000 miles so it is not a service that is required frequently. The correct service interval can be located in the owner’s manual or by inquiring of your service professional.

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Posted in Car Advice