There are different ways of determining when to replace vehicle tires. One is by the wear bars molded into the tires themselves in the tread, if your tread is down to the tread bar it's time for new tires. You can use a one cent piece,U.S., to determine, insert the one cent piece, penny, upside down, Lincoln's head facing down, into the tread, when the top of his head is equal at the top of the tread, replace the tire. The other is by age and I've heard that the government may set a standard, when your tire starts developing dry rot cracks it's time to replace the tire. I've heard that the government is getting involved in establishing an age to tires to replace them and having tires dated to help with that issue, I believe it was mentioned, something like 4 years old and time to replace. But different issues could arise depending on circumstances like just what kind of care did the tires receive. Tires on vehicles left out in the weather all the time, subjected to the sun and weather conditions would not last as long as tires on a vehicle kept in a garage all the time, and location of the vehicle could have bearing on how tires would hold up. So it's still up in the air. Also, has any of your tires had to have repairs done to them, patches or plugs installed because of road damage, nails and such. Very much a subjective subject. The way a person drives can also have a bearing on how long a tire will last as well as the wheel alignment of your car. How well you keep your tires inflated to specs also plays a part.