Who are these guys trying to kid?
We already have a de facto tax on mileage. It's called the "gasoline tax" and it's collected at every public pump in the entire state. You can't run most motorized vehicles without gas. The big rigs eat a lot more gas than the little ones, so it is somewhat proportional to weight.
If these clowns need more money, they can just raise that tax and do it without further individual modifications or useless bureaucracy - and face the wrath of the motorist while doing it. Instead, they want this fancy-pantsy new system which anonymously mandates Big Brother for everyone.
There must be an alternative "something else" up besides tax revenues for roads, but for some reason these guys don't want to tell us what "it" is!