The way your tires wear is a good indicator of other parts of your car. Abnormal wear patterns are often caused by the need for simple tire maintenance, or for front end alignment. Tires should be inspected at every opportunity; once a week isn't too often. Learning to read the early warning signs of trouble can prevent wear that shortens tire life or indicates the need for having other parts of the car serviced. Tires should be inspected 3 ways. First, visually examine all 4 tires; second, feel the tread by hand to detect wear such as feathering and third, check all 4 tires with a pocket type pressure gauge.
[td]Over Inflation Excessive wear at the center of the tread indicates that the air pressure in the tire is consistently too high. The tire is riding on the center of the tread and wearing it prematurely. Many times, the "eyeball" method of inflation (pumping the tires up until there is no bulge at the bottom) is at fault; tire inflation pressure should always be checked with a reliable tire gauge. Occasionally, this wear pattern can result from outrageously wide tires on narrow rims. The cure for this is to replace either the tires or the wheels.[/td][/tr]
[td]Under Inflation This type of wear usually results from consistent under inflation. When a tire is under inflated, there is too much contact with the road by the outer treads, which wear prematurely. Tire pressure should be checked with a reliable pressure gauge. When this type of wear occurs, and the tire pressure is known to be consistently correct, a bent or worn steering component or the need for wheel alignment could be indicated. Bent steering or idler arms cause incorrect toe-in and abnormal handling characteristics on turns.[/td][/tr]
[td]Feathering Feathering is a condition when the edge of each tread rib develops a slightly rounded edge on one side and a sharp edge on the other. By running your hand over the tire, you can usually feel the sharper edges before you'll be able to see them. The most common cause of feathering is incorrect toe-in setting, which can be cured by having It set correctly. Occasionally toe-in will be set correctly and this wear pattern still occurs. This is usually due to deteriorated bushings in the front suspension, causing the wheel alignment to shift as the car moves down the road.[/td][/tr]
[td]One Side Wear When an inner or outer rib wears faster than than the rest of the tire, the need for wheel alignment is indicated. There is excessive camber in the front suspension, causing the wheel to lean too much to the inside or outside and putting too much load on one side of the tire. The car may simply need the wheels aligned, but misalignment could be due to sagging springs, worn ball joints, or worn control arm bushings. Because load has a great affect on alignment, be sure the car is loaded the way it's normally driven when you have the wheels aligned; this is particularly important with independent rear suspension cars.[/td][/tr]
[td]Cupping Cups or scalloped dips appearing around the edge of the tread on one side or the other, almost always indicate worn (sometimes bent) suspension parts. Adjustment of wheel alignment alone will seldom cure the problem. Any worn component that connects the wheel to the car (ball joint, wheel bearing, shock absorber, springs, bushings, etc.) can cause this condition. Worn components should be replaced with new ones. The worn tire should be balanced and possibly moved to a different location on the car. Occasionally, wheels that are out of balance will wear like this, but wheel imbalance usually shows up as bald spots between the outside edges and center of the tread.[/td][/tr]
[td]Second-rib Wear Second-rib wear is normally found only in radial tires, and appears where the steel belts end in relation to the tread. Normally, it can be kept to a minimum by paying careful attention to tire pressure and frequently rotating the tires. Some car and tire manufacturers consider a slight amount of wear at the second rib of a radial tire normal, but that excessive amounts of wear indicate that the tires are too wide for the wheels. Be careful when having oversize tires installed on narrow wheels.[/td][/tr][/table]
Measuring Tire Wear
Tires have wear bars (flat spots)in the tread grooves to visually indicate wear. If the tread is worn down so the wear bars are flush with the surrounding tread, the tire is worn out and needs to be replaced. If you see cords showing through the rubber, the tire is unsafe to drive on and is on the verge of failure. Replace the tire without delay! The same advice goes for any tire that has bulges, deep cracks or the tread is separating from the casing.
[td]Tread wear can be measured using a penny. Place the penny with Lincoln's head upside down in a groove between the treads. If you can't see the top of Lincoln's lead, the tire is okay and still has some wear left in it. But if the top of Lincoln's head is flush with the tread, the tread depth is 2/32-inch (1.6mm) or less, indicating the tire is worn out and needs to be replaced.[/td][/tr][/table]
Some experts now say the same test should now be done with a quarter. If the top of Washington's head is flush with the tread when you place a quarter upside down in a groove, the tread depth is 4/32-inch (3.2mm). Though the tire still has some tread wear left, braking, traction and handling are significantly reduced on wet pavement compared to a tire with more tread on it. Because of this, many experts now recommend replacing tires when the tread depth is worn down to 4/32-inch or less if you drive in an area that receives a lot of rain or snow.
Tire tread wear can be measured with a simple gauge like this.
Other Mechanical Issues
Regardless of the position on the vehicle, a worn or loose wheel bearing will result in a severe irregular tread wear pattern. Typically, a series of flat spots appear across the tread face, from shoulder to shoulder, around the circumference of the tire. They can be straight or diagonal, and may be accompanied by cupping or scallop wear. Rib depression can also be caused by a worn or loose wheel bearing. In many instances, vehicles with high-speed empty hauls aggravate irregular wear patterns caused by bad wheel bearings. Flat spots caused by frozen or improperly adjusted brakes are usually easy to recognize. They are relatively uniform and extend evenly across the tread face. In dual applications, the flat spots will be in identical positions on both tires. Once again, this problem will not correct itself in service. In fact, as the tire “skips” every time it rotates, it creates another flat spot, which eventually covers the entire tire. New tires with severe brake skid damage repaired by building up the worn area with extruded rubber, curing the tire like a retread, buffing it to make it round again, and skiving out the original tread pattern! It doesn’t look pretty. Mis-mounted or out-of-balance tire/- wheel assemblies can also cause irregular tread wear patterns. These are characterized by both uniform and erratic depressions and are generally present around the circumference of the tire. In some instances, the problem can be corrected so the tire can be run out. In extreme cases, though, the tire must be re-treaded or scrapped.
It is literally impossible to describe all of the reasons why a tire develops uneven tread wear. There are so many variables related to alignment, balance, inflation, suspension and even driving routes that tire technicians could not diagnose every problem without some outside assistance.
this information was gathered from grumblemarc on honda-tech