Tires: Replacing Tires, cold climate, tire industry

Question
Should I replace tires based on age? If so,how old.  I purchased new tires for my 528 BMW in 2005 and they have 22,000 miles on them now.  My owner's manual,I recall, recommends replacing tires older that four years old.

Answer
Rufus,

Recent bulletins from the tire industry indicate that tires degrade simply due to time.  The age of a tire is important even if the tire is unused.  There some disagreement over how to best express this age limitation, but my take is:

If you live in a hot climate (AZ, CA, NV, TX, and FL) then the limit is six years.  If you live in a cold climate (MN, ND, WI, MT, etc), then the limit is 10 years.  States in between are  ..... ah ........ in between.