It signal´s generally that you have 25-30 miles left; I´m not sure if this means that it figures out your gas mileage and then alerts you of if it´s a preset amount of gas left in the tank. As a side note, if you can help it, don´t let your gas tank get this low. When it does, you start pulling any sludge or impurities off the bottom of the tank, as the junk PPM goes up with less and less gas.