Anything over 4 years old is working on borrowed time. The worst tires in that class are spares because they aren't being used. Rubber actually self lubricates itself when rolling due to heat. It doesn't matter if the tire is stored in the shade with no air, it will dry out. Ever wonder why trailer tires tend to blow out more than anything else we own?? Most only get used a couple times of year. They look great because they have full tread, but the rubber has dried out and is more prone to massive failure (blowout).
I've even had a tire in my wifes Jimmy blow out in the spare tire location which is inside the vehicle. It did a slow unravel but it was literally in pieces under the cover. It was the original tire (1985) that came on the truck and the original owner just used it as a spare when he replaced the original Tiger Paws. It was well worn and finally blew out after 13 years, but inside the vehicle on a spare tire mount?? It can happen!! Also, never buy used tires for the same reason. You're so much better off with cheap new tires than old dried out expensive tires. Technology of today is much better than even 5 years ago. I'm in the tire equipment business and the above info comes for tire engineers and consultants that live and breath this stuff. Hope this helps.
George
