[cptskippy]

I think you're missing the point. The tax goes towards infrastructure maintenance. All vehicles damage infrastructure and the amount of damage a single car imposes is a function of their weight and miles driven.

From an environmental view, the tax would be for cleaning storm water run off which contains contamination from the vehicle itself (e.g. oil, brake dust). From that perspective an older car is more likely to polute storm water than a new car.