Two tires? Four tires? What about just one?
I've received multiple emails lately questioning why customers are being told they need to replace all four tires or even two tires when only one of their tires is actually bad.
All-Wheel Drive (AWD) is the common denominator when it comes to "requiring all 4 tires." Yes, if your car is AWD the tires should be replaced in sets of four matched tires. An AWD vehicle's drivetrain is designed to do just what it implies: drive or apply power to any or all of the four wheels at any time. AWD greatly improves traction in the rain, snow or any other slippery conditions and provides for even greater performance and traction in dry driving conditions.
On AWD vehicles, the front and rear axles are connected by a driveshaft to a transfer case or viscous coupling. It's the transfer case's job, along with a computer in some cars, to apply power to any of the four wheels at any given time. This is determined by the amount of slip or wheel spin on any tire when compared to the other three. For example, if you're on ice and the left front tire starts to slip, it will be spinning faster than the other three the computer will sense the loss of traction. When this happens the car will reduce power to that spinning wheel and apply more power to the other three. That is exactly how the system is designed to work.
Having mismatched tires can trick your car into thinking there is loss of traction and wreak havoc on the AWD components. Forcing the transfer case and viscous coupling to work all the time will cause excessive heat and wear until there is a failure. If this is happening, you experience a binding or bucking feeling on tight turns, chirping tires or difficulty steering at low speeds.
Specifications vary between manufacturers: Audi says the rolling radius of all tires must remain the same or measure within 4/32 inch, Subaru says all should be with in a .25 inch circumference, General Motors say all tires should be with in 2/32 inch. Even brand-new tires can vary in size. At my shop, even when installing four new tires, we use a stagger gauge and measure the rolling radius of each tire to ensure the tires are matched to avoid problems.
In some cases you can replace a single tire, especially if they're expensive! If the other tires on your car have good tread remaining and are in good condition, we may consider having a new tires shaved to match the tread of the other tires.
On front- and rear-wheel drive cars, replacing single tires or pairs of two is fine in most cases.
• Try to avoid mixing brands or tread patterns. Can you imagine how goofy you would walk wearing a running shoe and a heel? Maybe your car won't feel goofy, but it sure will handle and drive differently with mismatched treads.
• New tires should always be installed on the rear axle.
• Never install a tire with a lower speed rating than specified by the vehicle manufacturer.
• Check tire pressure monthly.
• Rotations should be done every 5,000 to 7,000 miles.
• Always refer to your owner's manual for specifics about your car.
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