Tiny flakes of plastic, generated by the wear and tear of normal driving, eventually accumulate in the soil, in rivers and lakes, and even in our food.
I can't imagine much microplatics are getting chipped off of them. The tires have thousands of pounds of pressure being put on small surface areas when you round corners, where as a plastic bottleneck can dolphin into the water if hit by a large wave and not nearly as much friction placed on it.