Enough to wear the treads down.
What's interesting is that the thickness of the layer deposited as it rolls (from this simplified model) 1.5E-3/(50E60.250.8)=3nm which is about 0.15nm or a monolayer.
Let's say there are on the order of 100,000,000 cars in regular use in America. That's 300,000,000 liters of of plastic waste generated each year?
That's hundreds of thousands of tons of waste thrown into the environment every year! I had no idea that car tires shed that much.
That's like finding 20 empty two liter bottles just lying there. (Based on a bottle using 50 grams of plastic.)
So yeah, seems like a lot to throw out every year from every car.
Put another way, if every driver threw an empty two liter bottle on the side of the road every two weeks, you'd have an equivalent amount of plastic being shed into the environment.