Bullets should definitely work off screen, but I was thinking there could be a way to know there was a gas station near by, so you can slow your shooting down to avoid blowing it up. Either a loud warning when you hit one (maybe give them a little more hp to give you time to correct), or another indicator showing the nearest station.
For the extra fuel, maybe keep the same ring, but have look past 100% in a different color? Like reserve fuel or something. Not sure if that makes sense, but I recommend having something showing you're getting more fuel.
I meant the game itself hard crashed to desktop when I hit the exit on Demo 3.
Sorry I answered these backwards.