I know that here in WA they are proposing replacing the state gas tax with a mileage tax for the reasons you cite above.
As far as the impact being greater on rural vs urban drivers, I don't see it. If you live rural and drive lots of miles you're paying X per mile in gas taxes per mile on the gas you're buying anyway. For example: if the gas tax is 40 cents a gallon and you get 20 mpg, you're paying 2 cents tax every mile you drive. So if they replace that with a 2 cents a mile tax you're still paying 2 cents tax every mile you drive.
The same math applies whether the miles are driven in the city making 5 mile round trips or if they are driven in the country making 50 mile round trips. 2 cents a mile is 2 cents a mile either way. So I don't get the argument that it discriminates against one demographic or favors the other. I guess there is a slight penalty for rural drivers because the same car gets better mileage driving on rural highways than it will in the city. So the rural driver pays a tiny bit less per mile in gas tax than they would with a per-mile tax, but the difference is really small since the mpg difference between city and highway is generally only about 10% and in the real world both rural and urban drivers will be getting something in between city and highway mpg since they will be doing "mixed" driving.
Now, if the same tax rate is applied to all vehicles, then it actually penalizes people who drive economy cars because now they get charged the same tax per mile as the guy driving a gas guzzler. If you drive a car that only gets 10 miles per gallon then the 40 cents per gallon tax example above costs you 4 cents per mile, and replacing that with the 2 cent per mile tax from the example above actually means you pay less road taxes. That's the opposite of a per gallon tax that gets lower per mile the more miles your vehicle gets per gallon. Since the higher mpg vehicles are lighter they theoretically create less wear and tear on the roads, so in all fairness, they really should pay less per mile in taxes, so THAT part of the tax-per-mile system seems kinda backwards.
Where it evens things up is that the electric cars finally get to help pay for the roads, instead of getting a free ride by not paying any gas taxes at all.
The only way I can see this being made fair to everyone is for your per-mile rate to be tied to the vehicle weight. Heavier gas guzzlers continue to pay more per mile, lighter gas sippers pay less per mile, and electrics pay a rate per mile equivalent to a gas vehicle of comparable weight that would create a comparable amount of wear and tear on the roads.
The big issue is the tracking, but even that can be resolved pretty easily. Modern cars all have an function in the computer that tells how many miles are on the odometer. In areas with emissions testing (most cities) you have to go to an emissions testing facility for a test to renew your tabs. The same type of system could be used to determine mileage every year. They don't need to know WHERE you've driven - just how many miles you've driven, and the tax bill gets based on that annual miles driven.
In WA the vehicle emissions requirements law just phased out because it had a 20 year sunset provision and we met the emissions goals. BUT when it was in effect whenever you bought a car you had to get it emissions tested to get the new tabs transferred into your name. Under a tax-per-mile system they could do essentially the same thing and you would have to get the vehicle mileage checked and recorded when you SOLD the car - to pay your year's worth of mileage taxes before someone else assumes ownership and responsibility for future mileage taxes. They were already recording mileage every time you got an emissions test, and of course they record mileage every time the vehicle is sold at the point of the title being transferred.
Under a system like that the reduction in privacy issues would be pretty much nil, taxes would be fairly apportioned, and even the electrics would pay their fair share. Of course that means they would never implement a system that way.
