This is a nice article that highlights some of the more important case law regarding privacy while traveling in your car. But I think it's important to make clear that the Supreme Court has ruled that individuals do not have a reasonable expectation of privacy in their public movements, car or otherwise (U.S. v Knotts). While most agree that the infallibility and general capability of GPS does create a new technological perception of privacy in public spaces, and that an individual does not shirk all expectations of privacy once they are in a vehicle, one argument is that the technology does not make something that was previously public now private. Carpenter v. U.S. is a landmark case right now that will be decided this summer and is distinct from Jones in that there was no common law trespass to help the court answer the question about tracking via GPS or cell phone records.