A new set of terms and conditions for OnStar, the General Motors-supported safety and connectivity system, is generating privacy concerns among some subscribers, many of whom have long raised questions about the potential for abuse. For years, some G.M. car and truck owners have posted elaborate instructions to Internet forums and user groups detailing how to disable built-in OnStar equipment to prevent any tracking of their vehicles.

Last week, OnStar sent e-mails notifying customers of changes in the company’s policies. Some customers say two changes in those terms were of particular concern from the standpoint of privacy.

The first regards what happens when a customer cancels the service. Until now, when OnStar service stopped, so did the vehicle’s two-way communications system. As of Dec. 1, however, that will not necessarily be the case. Vehicles of owners who no longer subscribe could still be monitored via the system’s still-active two-way cellular link.

The second policy change concerns the potential use of the data collected by OnStar, which includes information like the vehicle’s speed and location, current odometer reading, driver seat-belt use and air-bag deployment. Under the new terms, OnStar reserves the right to share that information with other companies and organizations, even data culled from motorists who no longer subscribe to the service but who have left the two-way communications connection open.

“But the customer has a choice,” Vijay Iyer, an OnStar spokesman, said in a telephone interview. Under the new terms, a canceling subscriber can request that the two-way connection be shut off, he said. Customers who have already canceled the service are no longer connected via the two-way link, he confirmed.

As a default position, however, OnStar maintains the cellular connection after the subscription is canceled, a practice intended to collect data that could be useful to municipalities, for example, trying to improve traffic flow or to first responders attempting to determine the severity of a crash before arriving at the scene. Mr. Iyer said that there were related services that OnStar could offer in the future, like remote vehicle diagnostics.

OnStar has not started selling data that it collects, Mr. Iyer said. Under the new terms, data would be made anonymous and aggregated to allay privacy concerns. However, the open-ended language in the terms suggests that OnStar could, without reproach, sell the information to local governments, to companies offering location-based services and to marketers interested in directing messages to specific groups of drivers.

Jonathan Zdziarski, a forensic scientist, canceled his subscription based on the change in terms. His and other subscribers’ concerns range from invasive marketing techniques to unintended security breaches. Hackers, meanwhile, have proved how anonymous, aggregated data can be cross-referenced with other databases, for example, to identify individual users. Furthermore, law enforcement could issue subpoenas to grab data from an OnStar subscriber under investigation, a practice that is already commonplace with regard to portable navigation devices and cellphones.

The OnStar issue is the latest in a growing list of privacy-related concerns brought about by advances in onboard telematics systems. A Nissan Leaf owner recently uncovered a security hole that allowed anyone to tap into a vehicle’s speed and location information. Researchers at iSec Partners, a cybersecurity consultancy, have shown how a car’s wireless connections can be exploited, using a technique known as war texting, to break into a vehicle. Continuing to gather information about a car and maintain a two-way connection, some might argue, may leave owners vulnerable to new threats.

For OnStar, the issue is one of how to monetize this new world of customer data and services. The company is under increasing pressure from a new wave of free car-related apps for smartphones that mimic much of what has differentiated OnStar in the past. For its part, OnStar has not said to whom it would sell this information or how the data would be rendered anonymous, but as the backlash from customers has demonstrated, it must move carefully.