We are already taxed by the mile. That’s what a tax per gallon does — taxes per mile driven.
No, we are not. We are taxed by fuel consumption -- but only for certain fuels. There is a rough correlation between fuel consumption and miles driven, but it's not a direct relationship.
If Vehicle A is twice as fuel efficient as Vehicle B and they are driven the exact same number of miles, then the driver of Vehicle A will pay have as much in fuel taxes as the driver of Vehicle B -- regardless of how much these vehicles actually contribute to the capital and maintenance needs of the highway system.
There's an underlying motivation behind this discussion, and it's not all about tracking people, taxing them more, and all of that. What's happening here is that the nation's highway system is starting to be examined along the lines of a "public utility" business model, and not a "public space" business model. There is an enormous difference between these two, and this change in thinking is not a bad thing even if the means of collecting taxes may not be a good thing.
First and foremost, it is important for elected leaders and the general public to stop thinking about a road as if it were something that just gets built and then left alone until it falls apart. Instead, it needs to be viewed as an asset that has maintenance costs and long-term rehabilitation costs associated with it . . . and considerations for those costs need to be made when a road is built in the first place.