I think that the wear per distance is the only metric that makes sense, given these are modes of transportation. To be totally honest with you though, I only skimmed the thread up until your comment, and the statement about shoes caught my eye, so I had to ask. So any sources on shoe wear? I’m not even trying to argue, I’ve just had this question for months because I’ve heard others make the same claim that shoes pollute less than bike tires.