Navigation-and-emergency-services company OnStar is notifying its six million account holders that it will keep a complete accounting of the speed and location of OnStar-equipped vehicles, even for drivers who discontinue monthly service.

OnStar began e-mailing customers Monday about its update to the privacy policy, which grants OnStar the right to sell that GPS-derived data in an anonymized format.

Adam Denison, a spokesman for the General Motors subsidiary, said OnStar does not currently sell customer data, but it reserves that right. He said both the new and old privacy policies allow...