On page 13 of this issue of Real Answers, there’s a picture of an M726 radial that logged over 600,000 miles on its original tread. But if you look closely, there are some strange patterns in the rubber, especially on the left shoulder.

The tread appears to be discolored—in a wavy shape —near the middle of the shoulder rib. In fact, it’s not. You see different colors because you are seeing different rubber compounds.

Some manufacturers use just one compound for tire treads. To improve retreadability, Bridgestone "sandwiches" two different compounds. The layer that comes in contact with the road is made of tough, slow-wearing rubber that extends tread life and fights irregular wear. It’s called the "cap."

Beneath the cap, between it and the casing, there’s another layer of rubber, specially compounded to run cool and protect the casing from tread and pavement heat. Because heat is the number one enemy of casings, this "base" layer helps keep the casing cooler, for superior retreadability.

But when you get over 600,000 miles of wear, most of the cap layer is worn away, exposing the normally invisible base layer.

This is not a problem, especially since this M726 is ready to be retreaded anyway. So, there’s nothing wrong with this picture after all.