I ran across this on cracked.com and had to share it.  Why is it that the first half of your gas tank lasts a lot longer than you're bottom half?  It makes perfect since with the explanation of how the float in your gas tank works.Basically the gauge is a plastic bulb on an arm that floats in the gasoline.  When the bulb is at the top of the tank it is fully submerged in gas.  It takes a while for the bulb to move down, since the gasoline level needs to lower the distance of the bulb before it will move.  There's a great diagram in this article on cracked.com.  Also in the article, 7 other gadgets that lie to you everday!