[darkwizard42]

It seems a number of factors contribute to this beyond weight: how you drive (braking and accelerating aggressively), what conditions you drive in, the state of your tires, etc.

Weight seems to be one easy to understand and affecting factor. Why not start there?

[AnthonyMouse]

Because the thing you're worried about is microplastics, which are directly proportional to tire wear regardless of whether the wear is from heavier vehicles or more miles driven or idiots doing donuts, so if you just tax the externality directly you don't have to worry about which thing is causing it.