Tires: Tire cracking, ford sporttrac, cold climate

Question
I have a 2001 Ford SportTrac with Goodyear RT/S tires 255/70/16.  The tires have 75K miles on them but the tread is still well above the legal limit.  The tires are showing some cracks at the bottom of the tread (not on the sidewall).  How do I tell if the cracking is cosmetic vs functional?  I don't want to put 4 more tires in a landfill if they are still good.
Thanks

Answer
Ken,

Where do you live?

Recent annoucements from the tire compnaies say there is an age limit to tires.  My take on this is that if you live in a hot climate (AZ, CA, NM, NV, TX, and FL), then the limit is 6 years.  If you live in a cold climate (MN, ND, WI, etc.) then the limit is 10 years.  States in between are ......uh...... in between.

So if the time limit is enough for you to decide what to do - Good!

But judging the severity of rubber cracking is not easy to do without looking at the tires, so I suggest you have someone look at them.