1-) Bulges and Blisters on the Tire: Sometimes the outer surface of the tire begins to weaken. The result can be a bulge or blister that extends outward from the rest of the surface. This is similar to an excessive swelling in one of your blood vessels and you know that if your doctor tells you that you have got swelling in the wall of an artery, you’d better get to the hospital as quickly as you can before you blow out an artery. It’s the same with your tire. This weak spot can cause a sudden blow out, and if you don’t admit your car in its hospital (i.e. garage or service-station) before this happens, it may end up putting you in the hospital when the tire blows out on the freeway. So keep your eye on those tire bulges and blisters.
2-) Cracks in the Sidewall: Not all problems with the tires are going to be in the tread. They can also appear in the sidewall. Fortunately, it’s easy to do a visual check of sidewall problems. Look for tracks or cuts in the sidewall — grooves that are distinct enough to be visible to the naked eye. This could be a sign that your tire is developing a leak (or worse, that it’s nearly ready to blow out). This is definitely something you want to avoid. So if the cracks in the sidewall are starting to look serious, get that car to a repair shop at the next opportunity and start talking about getting them replaced. Better safe than sorry, as they say.
3-) Tread Depth: The tread on your tires should never fall below 1/16 of an inch (1.6 millimeters) in depth. If you regularly drive on slick, wet surfaces, you’d be even better off with twice that much. You can buy a gauge to measure the tread depth the way the professionals do, or take your car into the mechanic and ask about getting a new set of tires.
4-) Too Much Vibration: A certain amount of vibration is sure when driving, especially on poorly paved roads, but if you’ve been driving for a while, you probably know how much vibration feels right and how much means that something’s going wrong. There can be any of a number of causes for the vibration — maybe your tires are misaligned or unbalanced, or your shock absorbers are starting to go. But it could also indicate that there’s some sort of internal problem in the tire itself. Even if the tire isn’t the root cause of the vibration, the vibration could damage the tire and pretty soon you’ll have a problem. So if your car has a bad case of the shimmy-shimmy shakes, especially if you notice this when you aren’t driving on bad roads, take it to the mechanic right away to have it checked out. Too much vibration is almost always a sign that something is wrong.
5-) The Tread Wear Indicator Bar: Newer tires have a convenience that older tires lacked. They have tread wear indicator bars built into the tires themselves. These bars, invisible or barely visible when the tires are new, gradually begin to appear as the tread wears down. They appear as flat rubber bars running perpendicular to the direction of the tread itself. If more than one or two of these are visible on a tire, the tread is getting low. This should be particularly obvious in the wet tracks that your tires leave after you drive through a puddle. If the bars are starting to appear on any or all of your tires, it’s once again time to check with your mechanic or local tire dealer to see about getting your current tires replaced.