No, I mean just charge based on the odometer reading. Don't offer exemptions. The state where the car is registered collects the tax for every mile driven. It would lead to some mistakes in the small scale (what if I live in northern Oregon but do most of my driving in southern Washington, or vice versa?) but it seems like a serviceable alternative for the macro-scale problem of how states can replace the funding from gas taxes. And it does so without the expense or privacy invasions of schemes like automatic plate tracking or in-car GPS tracking.