silica is a compound make-up that makes tires grip well in the rain or wet circumstances. tires containing more black carbon work better is dry situations. a few manufatures like michelin and their pilot sport a/s use both. the very center of the tire is contains the most silica that a tire can contain by government regualtions. on the outmost portion of the tire, they have used the most black carbon possible. brigestone has a lot of information on the matter.... www.bridgestoneusa.com (i think)