The gas light comes on to remind you that the fuel is getting low, not that you are empty. If it was empty your engine would have stopped and your girlfriend would be walking to the nearest gas station for you!
So, 18.5 gallons - 14.5 gallons = 4 gallons is still in the tank. There is about 1-2 gallons of unusable fuel that will sit in the bottom (the pump doesnt suck fuel from the bottom in case there are sediments in there that would clog the system).
Now this leaves about 2 gallons of usable fuel for you, so @ 18 mpg you still have at least 36 miles left before you must get gas. If you want, you could easily keep driving until you have done 256 miles on the trip meter.