I'm in the every three year camp. Just too dangerous on the freeway and tears up the trailer. The 16 inch, load range e tires that systematically blew, had less than 3,000 miles on them, but were 7 years old. They also sat outside on the ground, so the ozone theory very well might apply. (I need to get some pavers for the tires to sit on). The oem replacement rule of thumb, is 6 years.