Car renters beware: Big Brother may be riding shotgun.

In a case that could help set the bar for the amount of privacy drivers of rental cars can expect, a Connecticut man is suing a local rental company, Acme Rent-a-Car, after it used GPS (Global Positioning System) technology to track him and then fined him $450 for speeding three times.

The case underscores the ways that new technologies can invade people's privacy, said Richard Smith, chief technologist at the not-for-profit Privacy Foundation.

"Soon our cell phones will be tracking us," he said. "GPS could be one more on the checklist here. Frankly, giving out speeding tickets is the job of the police, not of private industry."

Rental car companies have used GPS devices since the mid-1990s, installing systems to give drivers directions while they're on the road. "Fleet management" companies such as AirIQ and Fleetrack are also selling newer tracking services that help companies monitor their vehicles.

The New Haven Small Claims Court case pits New Haven resident James Turner against Acme. Turner also filed a claim with the Connecticut Department of Consumer Protection.

Turner paid for the rental car with a debit card last fall and, after returning the car, was shocked to find that an extra $450 had been taken out of his account, according to an article in the New Haven Advocate, where the case was first reported.

Turner could not be contacted for this article, and his attorney did not return phone calls.

When Turner contested the charges, Acme was able to point out on a map exactly where he exceeded the company's threshold speed of 79 mph.

For Acme, however, the policy is not about penalizing customers but about protecting its cars, said Max F. Brunswick, the attorney representing the company.

Acme recently decided to equip its cars with GPS technology and uses tracking services from AirIQ to find stolen rental cars and charge customers for "dangerous" conduct. The policy is stated in bold at the top of the rental agreement, Brunswick said.

"You have a problem in rental cars that people don't treat them like their own cars," Brunswick said. "The main reason to put in the GPS receivers is not to track the people but to track the vehicles. With this device you can track within a city block anywhere in the world."

That's not all that GPS and AirIQ can do. Calls to Acme itself were not returned, but information on the company's Web site promotes the service's ability to track the vehicle's location, notify the company when the car has crossed into another country or state, alert for "excessive speed," and even disable the car remotely.

Other car companies and vehicle monitoring services have embraced GPS as well. General Motors' roadside assistance service, known as OnStar, uses GPS to locate subscribers when they call for help. The company expects its subscriber base to climb to 4 million by 2003.

However, both GPS and cell phone technologies have raised privacy concerns.

"The challenge right now is to ensure, before these services and capabilities are widely deployed, that rules are in place," said David Sobel, general counsel for the Electronic Privacy Information Center in Washington, D.C.

At present, both Turner and Acme have left the decision in the hands of the Department of Consumer Protection. The judge in the small claims court case has delayed hearing the claim until the department has issued a ruling.

Brunswick said Acme plans to abide by the Department of Consumer Protection's ruling. "If they say it's not a fair practice, we will give him his money back," he said. "We are not out to make money on this."