Rubber literally grows on tress, and we're like no, plastic please.

According to this page it's about that 25% of the whole tyre, where more than half the tyre is not rubber/synthetic rubber but other stuff.

So there is more synthetic rubber than natural rubber. But the mind-blowing thing for me here is that I kind of assumed the whole tyre was synthetic, but they are only 25% plastic and still are the biggest source of mocroplastic.