Originally Posted by 79pmooney

That's a little like asking what kind of fabric is a red shirt. Some tan sidewalls are little more than the cotton or silk casing.with a little perhaps shellac. (I'm guessing 100 years ago, this was most tires.) Some are dyed rubber/butyl. Some are a near transparent coating over a tan colored synthetic cord. If you want a definitive answer, you have to specify the tire.