Taxing Mileage Instead of Gasoline

Oregon, USA is considering taxing cars by mileage instead of by gasoline use:
http://www.huffingtonpost.com/2009/01/02/oregon-exploring-idea-of-_n_154902.html

Oregon is among a growing number of states exploring ways to tax drivers based on the number of miles they drive instead of how much gas they use, even going so far as to install GPS monitoring devices in 300 vehicles. The idea first emerged nearly 10 years ago as Oregon lawmakers worried that fuel-efficient cars such as gas-electric hybrids could pose a threat to road upkeep, which is paid for largely with gasoline taxes.

It seems like a serious abuse of privacy. Who wants the Goverment tracking your vehicle's every move with GPS?

I thought the comment about taxing tires was a better idea than GPS tracking:

Why not just tax tires? A set of tires wears out about every 40K miles. Cars will use up tires no matter what their power source or efficiency. A tax of $25 per tire would generate exactly the same revenue as a tax of $.0025 per mile and both the privacy issues and the overhead costs of GPS would disappear. Or does somebody in state government own stock in a GPS company?????