Tread wear indicator bars becoming visible, cracks developing in tire sidewalls, bulges or blisters appearing on tires, and excessive vibration are all signs listed in the document that tires need replacing. The document provides details on each of these warning signs, such as tread wear indicator bars appearing as the tread wears down and bulges being similar to an aneurysm and risking a blowout. It recommends checking tread depth with a penny, looking for cuts in the sidewall, and taking the vehicle to a mechanic if any issues are spotted.

1. 5 Warning Signs You Need New Tires
5.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, but there's
an old trick that will give you a rough idea of how much tread depth you have left and it
won't cost you more than a penny.
In fact, it requires a penny. Take a Lincoln-head penny, the kind you find in your change
every day, and insert Abe's head (head-down) into the tread. If Lincoln's entire head
remains visible, you don't have enough tread. Take your car into the mechanic and ask
about getting a new set of tires.

2. 4.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. Use the penny test described on the
previous page to double check the depth, but 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.
3.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. 2.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 aneurysm in
one of your blood vessels and you know that if your doctor tells you that you have an
aneurysm, 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 put the car in the hospital (or service center, as the case may be) 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.
1.Too Much Vibration
A certain amount of vibration is inevitable 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.
Source: http://auto.howstuffworks.com/5-warning-signs-you-need-new-tires.htm#page=5
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