The problem with license plate readers is that there are only so many cameras out there. How can they know where everybody was all the time?
The answer is the Vehicles Miles Traveled tax. Many states and the federal gov't have proposed over and over that all cars have GPS trackers in them that tax them on how many miles they drive. They say "the problem is cars are more efficient, so we don't make as much money." (Can't you just raise the rate then? wtf?) or that this is "more fair", everybody is charged the same amount for how far they drive; as opposed to how much gas they use and how much carbon they emit.
But, come on, the real reason is almost certainly to track where everybody went, all the time. If there is anything the Snowden revelations have demonstrated, it's that if there is any possible way to capture data on people, the government is going to do it. Anything you can imagine, and many things that you could never have imagined, are being done. If you want to believe that a GPS tracker that hooks up to a gas pump only sends one bit of information, well, I suppose you deserve what you get.