Gas Tax And Mileage Tax

According to Yahoo news, President Obama will not ok a plan to charge people a tax on the mileage they drive rather than a gasoline tax.  Washington may decide that, but it sounds like some of the states have other ideas.  

Idaho, North Carolina, and Massachuchuetts are looking at the idea of a mileage tax.  Rhode Island's governor is on the record as being opposed to the idea.  The premise of a mileage tax is that it would better support our crumbling infrastructure as Congress puts pressure on car manufacturers to make cars that get better gas mileage. 

OK, so as the consumer gets a product that theoretically costs less to operate, the state governments step in to eat up the savings.  Nevermind that many of the fuel efficient cars won't hold your family, your luggage, and anything you choose to take on a trip.  Americans seem to like big cars, but as gasoline became more expensive, we downsized a little (or at least some of us did).  I predict that as Congress takes over the auto industry, they will pressure Detroit into making even more smaller cars that Americans will not buy.  At some point, as our cars wear out, we will buy them, consume less gas, and the government will be right there to take any savings we may see!

I have two further comments on this ridiculousness.  Number one, where in the Constitution does it allow the government to track my mileage (has anyone in government read the Constitution?)?  Number two, does this mean there is no point in selling my Mustang for something fuel efficient?