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Here’s Why Your AWD Vehicle Needs Four Tires When One Is Damaged

One of your tires was damaged and cannot be repaired. Your tire shop says you need four tires, not just one. Here’s why they are correct.
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If you have damaged one tire and taken your all-wheel drive vehicle to a repair shop or tire shop, don’t be surprised to find out you need four new tires, not just one to replace the damaged tire. The reason usually has to do with your all-wheel-drive system.

Related Story: Here’s Why Your Tire Cannot Be Repaired

Reason One You Need Four New Tires When One is Damaged
One obvious reason that you may need four new tires when you take your vehicle into the shop for a single damaged one is normal wear. All of the tires on your vehicle may have been worn to the point that they are no longer safe.

Every tire has wear bars. These are indicators in the tread surface that show you when the tread has worn down to the point that the tires are in need of replacement. A mechanic can also use a tread depth tool to measure the tread. Bear in mind that if the tires wore unevenly, it is the low point that matters, not the high point of the tread.

Reason Two You Need Four New Tires When One Is Damaged
If your tires are not fully worn out, you may still need to buy four new tires when one is damaged and cannot be repaired. All-wheel drive vehicles use transfer cases and differentials to distribute power from to rear and side to side. If one tire has more tread than the other three this creates an imbalance. The differentials and transfer case try to make up for this by turning one tire slightly slower to match the rotation speed of the tire with more tread. Drivetrains are not designed to do this constantly and they can be damaged by having one tire with a lot more tread than the other three.

The solution is to replace all four tires, rather than just the damaged and unrepairable one. This can come as a shock to owners who may feel that the tire shop or mechanic is trying to upsell them on tires they don’t need. Refer to your owner’s manual for information on how many tires you need when one is replaced. Manufacturers refer to the amount of tread left on a tire as “degree of wear.”

What Do Experts Say About Replacing Four Tires vs. One?
We reached out to tire expert Jay Condrick from Boston Mobile Tire for an independent expert’s view. Jay told us, “A bigger than 1/4” difference in diameter between 2 tires on the driven axle of a 2wd vehicle or the 4 tires on an AWD vehicle will be overburdened the mechanical and or electronic differential between them - some manufacturers like Porsche are even stricter at 3/32”, so the definitive answer is always to check your owner’s manual.” Jay went on to add, “With new tires having typically 10/32” of tread new and 2/32” at end of life - as long as your tires are half worn or less you probably don’t need to replace more than one. About 75% of the time we replace just one.”

One Possible Solution - Tire Shaving
Since the issue is one new tire having more tread depth than the remaining three worn tires, Tire Rack offers a solution. This tire provider can supply you with a new tire with the tread shaved down. This can enable you to buy just one replacement tire. Before you call Tire Rack, ensure that your mechanic or tire shop has told you how much tread is on the remaining three tires, and get the full sizing, brand, and model information from the existing tires. That information is on the sidewall.

Tires are one of the most expensive maintenance and repair items on any vehicle. If you have a tire damaged, ensure that replacing just one is not prohibited by the vehicle’s specifications.

John Goreham is a long-time New England Motor Press Association member and recovering engineer. John's interest in EVs goes back to 1990 when he designed the thermal control system for an EV battery as part of an academic team. After earning his mechanical engineering degree, John completed a marketing program at Northeastern University and worked with automotive component manufacturers, in the semiconductor industry, and in biotech. In addition to Torque News, John's work has appeared in print in dozens of American newspapers and he provides reviews to many vehicle shopping sites. You can follow John on TikTok @ToknCars, on Twitter, and view his credentials at Linkedin


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Comments

We have driven Subaru's since 1987 and never did what author says to do. We never had a drivetrain problem. Take this one with large grain of salt.
When a vehicle turns the outside tires spin faster than the inside tires, so I don’t believe a tiny difference in wear patterns will blow out a transmission or differential. Are there ANY cases of this being the deciding factor in abnormal or premature failure of a transmission or differential? I really doubt it
Yes, every vehicle has a mechanical design to allow for the individual tires to move at different rates. Otherwise, tire scrub would not allow for normal driving. When we posted this to the Facebook Subaru Forester owners club, owners who had mechanical failures did reply. You can view the comments there if you wish. Here are some of what was posted: "I had to get a new cvt from cruising on unevenly worn tires. 3 extra tired is much cheaper, trust me." "Messed up my Volvo because of this."