When does my low fuel light come on

I know I have an 18 gallon tank. But whenever I fill it up it takes about 14 to 14.5 gallons of gas. But the dial has gone to the E and below. So my question is how many gallons should I have left when the idiot light comes on. And is this normal to be at the E or below but have 3 or 4 gallons of gas left. Probably just the gauge reading issue I have read about.