I've tested the waters with my old GT & my newer GTS...
The first bar gets me around 64 to 80 miles, and then once that bar goes (which tricks me into thinking I'm getting great milage), then it starts dipping fast & I maybe get 15 miles per bar.
...and I have gone quite far down on the tank to where I had to be running on fumes. I got it not to the last notch, but BEYOND the last notch to NO NOTCHES! Man, I freak when that happens, but it has happened a few times when I think I could make it to work & back with 3 notches, but then they slowly disappear & I run out.
...when I fill it to the brim after I reach no notches, I usually come up with about a gallon left (13.5 to fill the tank, sometimes a little more).
...I don't normally push it like that, but sometimes I had to make it stretch before pay-day or just didn't have time to fill.