Distance To Empty

When my Z says it has zero miles to empty, I have never been able to put more than 16 gallons in the tank. The owners manual says it has a 20 gallon tank so there should be 4 gallons left. Since I get around 28 mpg on the highway I should be able to get over 100 miles before it dies. I have never pushed it that far, have no desire to run out of gas or damage the fuel pump, but I have taken it over 50 extra miles with no problems. The range of the Z is really good, over 500 miles so I usually have to stop before it does.