I just replaced a pair of worn-out bike tires. They certainly wore out by the rubber going somewhere. The amount of rubber lost was maybe a couple mm thick, by

I just replaced a pair of worn-out bike tires. They certainly wore out by the rubber going somewhere. The amount of rubber lost was maybe a couple mm thick, by 15 mm wide. The tires had probably about 8000 miles on them. I don't know if that info helps with doing the math on where rubber particles primarily come from.