Because you have a gas gauge! Duh. When the little needle goes down towards the little red
E. You need to fill the tank. No need for a reserve.
You gave up a temp gauge and that reserve tank when you decided that the pregen was just to ugly to buy. So sucks to be you! Haha.
DIY:
Buy one of these...
Just bungee it to the rear seat and you've now got a reserve!