That freedom would disappear with a GPS-based mileage tax. In fact, since such systems would probably be accessible wirelessly, they might even allow authorities to locate you in real time, or be alerted immediately if you’re speeding. Promises that that capability wouldn’t be abused are likely to ring hollow to many. (They certainly ring hollow to me).
There are two responses to these concerns: One is that you don’t really have much privacy anyway. Authorities can already track you via your cellphone, which checks in with towers along your route even when you aren’t using it, and many localities are building up networks of camera-based license-plate readers that track every vehicle in the area. And vehicle informatics systems like OnStar also know where your vehicle is and can be accessed remotely, while plug-in devices like Progressive Insurance’s Snapshot keep tabs on your driving habits in ways that go well beyond location.
So, relax: You’re not losing your privacy. You’ve already lost your privacy!