fuel tanks are often slightly bigger than stated in the owner's manual. the fuel tank capacity also doesn't include the filler neck. the underestimated capacity, along with the fact that the gauge reads "E" while there are still a couple gallons left is used by manufacturers to prevent premature failure of the fuel pump. the risk that you run when running near empty, or completely out of fuel is that the high pressure fuel pump will overheat. since most fuel pumps reside inside the gas tank and are cooled by the gasoline flowing through and surrounding the pump, they can overheat and fail if there is not sufficient fuel in the tank. so running on "E" probably isn't something you want to do on a regular basis.

also, topping off the tank once the pump automatically shuts off isn't a good idea either. in addition to the environmental factors (spills, vapors, etc.) you can also saturate and damage the carbon filter that is part of your emissions system. not to mention that the fuel you are paying for can be recirculated back into the gas station's tank. if you've ever noticed that sometimes when you start filling up the pump wants to shut off a couple times before it flows properly - this is often the result of the residual vapors left in the pump from the previous customer.