so the route I'm taking is: top it off from the instant the indicator lights up and write down the exact gallons in the tank to top off. Then drive until it goes off again. I can divide the total miles driven to the number of gallons in the tank I wrote down. This would be an accurate assessment.
Side note, i noticed on the guage, the first 1/4 tank lasts a lot longer then any other quarter, almost like a 1/3.