It's a lot of unnecessary work, just significantly increase the gasoline excise tax

See the AP story, "Oregon looks at taxing mileage instead of gasoline." From the article:

... fueling the search for alternatives is the political difficulty of raising gasoline taxes. The federal gas tax has not been raised since 1993, and nearly two dozen states have not changed their taxes since 1997, according to the American Road & Transportation Builders Association.

In Oregon's pilot program, officials equipped 300 vehicles with GPS transponders that worked wirelessly with service station pumps, allowing drivers to pay their mileage tax just as they do their gas tax. Whitty said the test, which involved two gas stations in the Portland area, proved the idea could work.

Though the GPS devices did not track the cars' locations in great detail, they could determine when a driver had left certain zones, such as the state of Oregon. They also kept track of the time the driving was done, so a premium could be charged for rush-hour mileage.

The proposal envisions a gradual change, with manufacturers installing the technology in new vehicles because retrofitting old cars would be too expensive. Owners of older vehicles would continue to pay gasoline taxes.

The difference in tax based on mileage or on gasoline would be small - "pennies per transaction at the pump,"