I don’t have a reference for this, but I was told by my father that it dates back to the days when filling stations pumped gas for you and it was to help stop people from accidentally driving off before the pump jockey was finished. They didn’t have pop-off hoses then either, so driving off with the hose still in the filler hole could do damage all around.

As one of my employees always says, there are two kinds of countries in the world. Those that use the metric system and those that have put a man on the moon.