Here's a radical suggestion - why not make the car tags we have to buy annually BE the road tax - that's what these tags were originally created to pay for and I think they should return to that. Base it on the weight of the vehicle and that will be the tax for as long as the car is driven, regardless of its age.
Or add a road use tax to tires - the less a car is driven, the longer the tires last, therefore the less that person will pay in road taxes. A person who drives a lot will buy tires more often, thus paying more in road taxes. This will also be an incentive to take better care of tires (proper inflation, proper brake use, and all) which will reduce gas use and extend the life of the car as well as the roads.
Fuel is already at the maximum tax consumers can afford to pay. Adding an expensive-to-implement mileage tax on top of that when we already have these other, better, tax venues available. In order to implement a miles-driven tax, devices will have to be installed in cars to track those miles and then devices have to be installed on fuel tanks to calculate those miles and there's room for all kinds of defects and errors in this when simply adding a tax to tires - already rated for set numbers of miles - has far less room for errors and defects and if a tire is defective, the consumer has recourse which we don't have with these electronic or computerized devices added to cars and gas pumps.
Return the car tags back to being the road use tax it originally was and add a tax to the purchase of tires and problem solved - simply, inexpensively, and with very little room for defects and errors.
Even the electric cars have to buy car tags and tires, so this tax would be evenly applied to everyone - no cried of "unfair!"