Lure em into a house and set the house on fire!!! With lots of gasoline. BIG fireball!!!
Okay I know that some may object because you need that gasoline for your vehicles. But how long is it going to be until the tires on your vehicles - and in all the stores - start to decay to the point where they're not usable?
You're going to have to switch to horses sooner or later. Might as well start having fun with the gasoline.