Your tires produce traction by pressing against the road, causing the tread rubber to gradually wear away with regular use. A tire must be replaced before its tread area is worn completely smooth, as bald tires are unsafe to use in wet, snowy, and other slippery conditions.
Tread wear is measured by tread depth, or the depth of the tire's deepest grooves. As the tread wears down over time, the grooves become shallower and shallower. When the tread is worn down to the minimum tread depth of 2/32", the tire is considered fully worn and must be replaced. Your tires may have small "wear bars" molded into the bottom of their grooves that indicate when the tread is fully worn.
The softer its tread rubber, the faster the tire wears. All-season tires tend to have harder rubber and deeper grooves than summer tires, so they usually wear more slowly and last longer. That said, treadwear depends on many factors aside from the tread compound, including how aggressively you drive, your vehicle's load weight, and the road surfaces you drive on.
You can get the most mileage out of your tires by ensuring that each tire wears evenly over its tread area. Uneven treadwear means that part of the tread is wearing at a faster rate than the rest, so the tire wears down to the minimum tread depth after less mileage, cutting short its lifespan. When wear is evenly distributed across and all around the tread, the tire can run for longer before needing to be replaced.
Uneven tread wear is often caused by incorrect tire inflation, wheel misalignment, or other tire issues. Such issues not only wear out the tread faster, but can also pose a serious safety risk while driving. Irregular wear patterns serve as clues that can help you identify the underlying issues and have them fixed.
Overinflating or underinflating a tire prevents the full tread width from pressing against the road evenly, causing part of the tread to wear faster down the middle or along the shoulders. Setting your tires to the recommended pressure for your vehicle load ensures that each tire distributes wear uniformly across its tread.
Out-of-spec wheel alignment is another major cause of uneven wear. Incorrect camber causes a tire to lean to one side, pressing harder on the inside or outside shoulder and wearing faster on that shoulder than the other. Excessive toe-in or toe-out alignment points the tires at an angle relative to the direction of travel, causing the tread blocks to wear at a slant and giving the tread area a "feathered" texture. Proper wheel alignment ensures that your tires are positioned correctly on the road and wear evenly.
In addition, tires receive different amounts of wear depending on their axle position. Braking, steering, and acceleration forces put more stress on the drive axle than on the free axle. So if your vehicle is FWD, then your front tires will wear out faster than the rear tires; and if your vehicle is RWD, your rear tires will wear faster than the front tires. Rotating your tires regularly ensures that all four tires wear together and reach the end of their lifespans at the same time.