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Front tires on an all-wheel drive (AWD) vehicle often wear more quickly than those on the rear axle. Why? Misalignment, aggressive driving, under-inflated tires and failure to regularly rotate front tires to the back are common causes. (Get tips on avoiding some of these problems here.)

So it can be tempting to try to nurse along AWD tires that still have some tread life on them by replacing only two. But this is almost always not the best decision for your pocketbook. Here’s why it’s better to replace all four AWD tires at once.

MISMATCHING AWD TIRES CAN DAMAGE YOUR VEHICLE

A new tire is actually larger than one of the same brand, type and size that’s partway through its tread life. There's a measurable difference in tire circumference.

To understand why this matters, think of two horses of unequal size at gallop. The taller horse covers more ground in a single stride than the smaller horse. Over the miles, the smaller horse will take more strides and run harder to keep up.

The same is true for mismatched tires. A bigger tire (one with more tread depth) travels more distance in a single revolution than a smaller tire. So to compensate, a smaller-diameter tire will spin faster, revolving a bit more than one revolution for every revolution of the bigger tire.

Tires with less tread depth will revolve more times per mile than tires with more tread depth. Source: Tirecomparator.com Here’s an illustration showing how even if a tire is labelled the same size, the tire with less tread depth (the smaller/used tire) will revolve more times than the newer (bigger) tire with more tread depth.

A tire with 11/32 tread depth travels 11 feet further than one with 8/32 tread depth.

So why is mixing new tires with used on an AWD a potential problem? First, a difference in diameter of less than half an inch between front and rear tires on your AWD can mean trouble for the drivetrain, the parts of the car that transfer the power to move the vehicle forward.

Parts of the drivetrain Source: http://repairpal.com/drivetrain This is because the heat that builds up from the effort of the smaller tires “trying to keep up” creates stress on the transmission and axles that can result in expensive repairs.

Second, if the two tires on one axle are spinning faster than the others, your car’s electronics may think those are slipping, and put you in the wrong gear. Most AWD systems in today’s cars include sensors on each wheel that monitor traction and wheel speed hundreds of times per second. This is what allows the AWD system to work in slick conditions, by sending power to whatever wheel(s) have the most traction.

When smaller tires are spinning faster, the system may put your vehicle into four-wheel lock, the gear that’s used for driving in slippery conditions. That’s a no-no, since driving in that mode on pavement or at speed can cause damage to your car.

BUY TWO OR FOUR NEW AWD TIRES?

If two of your tires are due to be replaced (Hint: use the penny test to find out), it's absolutely essential to check your vehicle’s owner’s manual to see if it recommends replacing all four of your AWD tires at once. Look in the tires and transmission sections. Or call your auto dealer for your make and model and ask the service department. Ignoring this advice may result in costly damage to some of the most important driving components of your car, like the transmission. Any reputable tire dealer will follow what the car maker says to do. And almost all manufacturers agree that you should replace all four.

Also, get a tire pro to use a tread depth gauge to accurately measure the tread if you think two of your tires still have some tread life in them. If there’s more than 3/32nds difference in tread depth between your new and keeper tires, spring for a full set of four new tires. Even that much of a difference really matters.

RECAP: WHY YOU NEED TO REPLACE ALL FOUR AWD TIRES

Mismatched tires are often a primary factor in drivetrain damage. To minimize variances, always match tires: same brand, size and type. The tread depth difference between front and rear tires should be no more than 3/32nds of an inch. If it is, you’ve got a problem with tire circumference difference. Get all four tires replaced.

Read the full article here: https://www.lesschwab.com/blog/post/replace-all-4-tires-on-your-awd-vehicle