Next time it gets that low, fill it right to the top. Your fuel capacity is 18 gallons, so if you subtract what it takes to fill it right up you will know how much gas was left in the tank. If you put in 16 gallons, you know there was still 2 gallons left in the tank. Record your mileage.
Next time you need gas, fill it up one more time and use the amount of gas you put in and the miles since your last fill to calculate your miles per gallon.
If you are getting 20 miles to the gallon, and hav 2 gallons of gas left in the tank when you hit the mark on the gage you are referring to .....
Let'* talk about never filling your car up. That means you are fueling often and putting in small amounts. Why not fill right up and continue to fill often and putting in small amounts. Keep your tank at the top end, not the bottom. This will reduce condensation in the tank, and give you a reserve when you have a ways to go between gas stations. Other than the initial investment to fill it, it won't cost any more money to run it that way.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)