How much am I responsible for? If I weigh the tires (all 4) when I buy them, then use them for X years, then weigh them when I get rid of them for the next ones- then that is how much I responsible for. And I can divide it by the years I had it for a yearly number too.
And that is how much microplastic I would agree I am responsible for with the tires. There is also the carbon cost of making them, supplying them, and disposal. But we were talking about microplastics...