hehe, been there too.
i have allowed my light to go on a few times, and even let it "hit" the line, but ive never been able to put more than 23 gallons in when the light goes on. so i guess it still has about 3 or 4 gallons left in the tank, which supposeably is a 26 gallon tank on my 05.
either way, at 13mpg, i get gas real quick when i let it go that low, which is usually about $70 per fill up...
