A global positioning system can ensure your road trip stays on track, but it can also track you.

An Arizona man was stunned to find his bill had jumped up $1,800 after the rental car company used the GPS travel log to prove he crossed state lines -- which according to their contract, means a higher per-mile charge.

Robert Sinclair of AAA, says GPS information is being used in many ways.

"Not only are you able to find your way with these devices and track where you are, but if they are in rental vehicles, for example, they could very easily track where you've been and how fast you were going when you were there," Sinclair said.

In 2001, a New Haven, Conn., rental car company charged customers $150 every time they drove over 79 miles per hour for more than two minutes. The controversy over that situation continues.

"Legally, it was improper. The company could not legally track and fine people without providing some notice because the contract, in essence, was deceptive and misleading by failing to give them this crucial information," said Richard Blumenthal, the Connecticut attorney general.

Blumenthal believes consumers should have been told how the GPS system could be used and told clearly about the violation charges it would impose. The moral question is even more vague.

"I'm very troubled by a system that tracks people, literally, every moment of the time they use a car," Blumenthal said.

AAA advises consumers to ask about GPS tracking to avoid surprises when the bill arrives.

Consumer Alert spoke with several car rental agencies. Enterprise said that the GPS device is not part of their system. Hertz rental company said that their systems don't have the capacity to track consumers.

So, it all depends on where you rent the car. Some places may use the GPS system, and others do not.

The Consumer Alert bottom line is to make sure to ask for the contract and be sure to read the fine print.