They actually make a measuring device for checking tires called a tread gauge. This is a handy little tool that only costs a few dollars. It works a lot better than using a penny that may have been recommended in the past.
A lot of friends and family ask me when to replace tires. In my own personal opinion I believe it is time to replace tires when there is less than 3/32 of tread remaining. If the winter driving season was approaching I might up that to 4/32 seconds.
Some information about tires
Some of these road conditions would be a wet or dry asphalt road, a road made of concrete and of course in some rural areas there will be gravel roads or maybe no road at all which we can call dirt roads. It is the job of your tires to provide good traction under all of these different driving conditions.
As a tire rolls on the road friction is created between the tire and the road surface. This friction is what gives the tire traction. Although you need good traction it must be limited to a degree. Too much traction means there is too much friction. Too much friction means there is a lot of rolling resistance. Rolling resistance wastes engine power and fuel. Therefore the tire manufacturers must balance traction and rolling resistance so that the vehicle can achieve its maximum fuel economy without it sliding all over the road.
It is for this reason that I strongly recommend sticking with the factory recommended tire replacement guidelines. Not only does your owner’s manual tell you exactly what kind of tires to put on your vehicle but there is also a tag or sticker on the driver side door jamb of most vehicles that tells you what size belongs on that car. The size is important but also the speed rating of the tire.
On the Cadillac that I'd purchased recently which is a 2009 SRX model the rear tires are a different size than the front tires. It is important to maintain this factory recommended sizes so that the vehicle will handle as per design intent. This also means that tire rotation is not possible because of the different sizes.
A lot of people wonder when to replace tires and what gives tires their strength. It is the layered construction of the tire itself that provides the strength. It starts with the casing which consists of layers of rubber impregnated chords. Typically tires are made of two, four or eight ply construction. The amount of plies and the way they are constructed determines the tires overall strength, handling, ride and even the amount of road noise.
For the most recent automotive articles posted to this site this next link will take you to the online auto repair blog homepage from this page about when to replace tires.