But the tread wears faster. A small tire contacts the road proportionally more often as it rotates. The amount of rubber that gets shed is a function of a bunc

But the tread wears faster. A small tire contacts the road proportionally more often as it rotates. The amount of rubber that gets shed is a function of a bunch of factors including the contact area with the road, the weight of the vehicle, the speed of the vehicle, and of course the miles done. It's hard to believe that tire size will change any of that much within the range of options for a given vehicle, so particle emissions per mile are going to be about the same regardless of tire size.