2011 Elantra Gas Tank Size
 

So I was told by the dealer and the Hyundai website that the gas tank is 12.8 gallons. When my empty light is on and I still drive around for another 15 miles and fill my tank up full it only at maximum i've seen it fill up 11-11.8 gallons. So is there a reserve tank in this car?

Yes, when the light goes on there is usually about 2-2.5 or so gallons in the tank. All my Hyundais have been that way.

However, running the tank that low on a rugular basis is not a good idea. The electric fuel pump is in the tank and it uses the fuel to cool it. Running the tank that low causes the pump to run hotter, shortening its life.