Front And Rear Tires Wear Differently
Think about it. All that parallel parking. All those three-point turns. With each turn of the steering wheel, pressure is brought to bear on the front tires. (This is even more accentuated in front-wheel-drive cars, where the front wheels also supply the main motive power for the vehicle.) Resistance causes friction, which in turn produces heat. The result? The front tires wear quicker than the rears. Because of this, it's necessary to rotate the tires front-to-rear several times during their life cycle to 1) equalize tread wear and 2) maximize the life of the tires. This is what we refer to when we say "rotate the tires." Rotating generally does not refer to either of the following actions:
If your car has staggered wheels -- the front and rear tires are two different sizes, you can still swap the tires side-to-side, but not front to back. If
your tires are unidirectional (specific to one side of the car) or
asymmetrical (the tread pattern changes from the inside of the tire to
the outside), you can rotate them front to back but not side to side.
Also, if you happen to have staggered wheels that are also unidirectional
or asymmetrical, you cannot rotate your tires at all! But that's rarely
the case.
Here is an illustration to better understand the components of a basic tire:


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