AWD Tire Matching and Drivetrain Service

An AWD system constantly measures how fast each wheel turns. A new tire is larger in diameter than a worn one, so it turns slower. The system can read that difference as slip and keep the coupling or center differential working, which builds heat and wear.

Many manufacturers publish a maximum allowable difference in tread depth or circumference between tires. We measure all four and compare.

Options when one tire is damaged:

1. Replace one tire if the others are nearly new and within the allowable difference.
2. Replace one tire and shave it to match the others, where available.
3. Replace two or four tires to stay within spec.

Our patch versus replace guide helps decide whether the damaged tire can be saved first.

No. Some AWD systems run mostly front-wheel drive and engage the rear only when slip is detected. Others send power to both axles all the time. Some use a center differential, others use an electronic coupling.

Your owner's manual explains your system. The crossovers page covers on-demand AWD in more detail.

AWD vehicles may have a transfer unit (sometimes called a power transfer unit), a rear differential, and a coupling, each with its own fluid. Some are described as lifetime fills, but many manufacturers still list inspection or change intervals under severe use.

We look up your model, check the schedule, and inspect for leaks and fluid condition where access allows. See differential, drivetrain and 4WD service.

Very. Rotation keeps all four tires wearing evenly, which keeps diameters close. Skipping rotations on an AWD vehicle often leads to a situation where one damaged tire forces a full set.

Rotate on the schedule in your manual and check pressure monthly. Low pressure on one tire also changes its diameter. Our tire rotation and balancing service keeps wear even.

Most days, AWD does little in the Inland Empire. It earns its place on the first rainy day after a long dry spell, when oil on the road surface makes pavement slick, on steep wet driveways, and on winter trips to the mountains.

That does not make the system maintenance-free. A coupling that rarely engages still has fluid that ages, and seals still dry out in the heat. It also means tires matter more than the drive system in most conditions. AWD helps you accelerate, but it does not help you stop or turn. Good tread and correct pressure do that.

If you tow with an AWD crossover or SUV, stay within the manual's tow rating and ask about fluid checks, since trailer weight adds heat to the rear drive unit. The towing and hauling page has a pre-tow checklist.

AWD systems stay healthy when the tires stay even and the extra fluids get changed. Instead of a separate appointment, fold the AWD checks into visits you already make, so nothing depends on remembering an unusual interval years from now.

At every oil change or rotation:

1. Measure tread depth on all four tires and record the readings.
2. Compare pressures and set them to the placard.
3. Look for seepage at the rear differential and at the front power transfer unit, where equipped.
4. Mention any clunk, whine, or shudder you have felt since the last visit.

Once a year:

1. Inspect CV boots and driveshaft joints.
2. Check fluid condition at any fill plug the manufacturer allows.
3. Road test through tight parking-lot turns to feel for binding.

At the manufacturer's interval:

1. Change the coupling or center differential fluid.
2. Change the rear differential and transfer unit fluid.

Not every AWD vehicle lists each of these fluids, and some are labeled lifetime. On high-mileage vehicles we still like to look at their condition, because fluid that is never changed still ages with heat and miles.

Most AWD repairs begin with one of a handful of habits. They feel harmless because the vehicle keeps driving normally for a long time before a noise or warning appears.

Having made one of these mistakes does not mean damage is certain. An inspection tells you where the system stands today.