>I tried plugging these in and it still comes out as a 6-wheel commercial bus has several times the tire wear as a 4-wheel light truck, rather than being the same.

Pardon me if I ask the obvious question, but did you divide your result by the average number of people moved? Because that's the actual utility of mass vs. individual transport. I would find it rather surprising if tire wear was the one measure were buses didn't win out.