When The Gas Light Goes On

i don't try to drive my HH until the gas light goes on however it does happen soemtimes. this is what i am finding. my 07 HH has a 17.2 gallon tank. the gas light goes on and when i go directly to the gas station and fill it up, i can get around 14.5 - 15 gallons in. this means that there is approximately 2 or more gallons of gas in my tank when the gas light goes on. I usually average between 23 and 24 mpg, so my theory is that i should be able to drive at least 45 -50 more miles after the gas light goes on. although i would not try to go that far, but it is nice to know when i am on the highway and the gas light goes on, that i don't necessarily have to find a gas station instantly.