If you want to tax pollution, tax fuel consumption or actual tailpipe emissions, not a broad class of vehicles.

Which emits more -- a small family SUV or a BMW M3?

Governments have taxed vehicles based on weight, width, engine displacement, etc. -- and the automakers adapt. It's very easy to game the system. Just tax vehicles on a model-by-model basis on the fuel they actually use, and on what they actually emit.