Some GPS manufacturers are trying to make their devices more effective in locating the cheapest gas prices.

GPS units have listed gas prices for a while, but they usually were updated only every few days. Now, some are being updated on a four-hour basis.

Tom Murray, vice president for marketing at TomTom, said his company is trying to make what its units display more reliable.

``What we've seen since we introduced this feature is not only continued growth for TomTom but also a significant increase in subscriptions to the fuel service," Murray said, adding, ``It allows units to get information on the lowest gas prices available to customers, whether it's on their route or on the circumference of where they're driving."

The gas data comes from the government and is updated very four hours, with the updates actually sent to customers' cell phones.

If you have a Bluetooth compatible phone, the information will be sent over the Bluetooth phone directly to your TomTom and you can get the information effectively in real time.

Magellin and Garmin also offer gas price updates on some of their GPS models.