It all depends on the car. Since this is the PT forum I'll guess you have a PT, is it a turbo, GT, regular, auto or manual? Each one will get different millage and it will also depend on how heavy your right foot is. :) Our 06 base 5 speed regularly averages 29 mpg with a 15 gal tank and when the light comes on there is about 3 gal left. A good rule of thumb is to never let the tank get empty and run the pump dry, any sediment and or water that may be in the tank will get picked up and wreak havoc.