Aiming to cut use of gasoline and dependence on foreign oil, the federal government gives you a $7,500 tax credit if you buy a new electric car. Various states add their own tax credits ranging up to $5,000. So far, so good. But now state and federal proposals would penalize electric cars for not paying gasoline tax, by imposing a tax on the miles they travel. And here's a hackle-raising detail: to make it work, states would have to track your mileage with a mandatory GPS device on the car.