Mileage taxes will be used to tax people out of larger vehicles they like to drive.
Higher tax rates will be charged for heavier vehicles.
The government in their effort to prevent people from realizing they are paying taxes, (by charging it on bulk gasoline, instead of directly to the consumer) have prevented themselves from using their taxing power to social engineer people. They have now decided that it's more important for people to know it's going to cost them more to drive gas guzzling vehicles than it is to conceal the taxation.
That being said, IF the mileage tax replaced the gas tax, and IF the mileage tax was charged to everyone at the same rate, including bicycles and walkers that use the road, and IF it's charged to each person riding a bus, I would support it.
NEVER, NEVER GPS except for public government run buses, to determine the charge to the rider.