We are already rationing. Oil has a price. This makes it too expensive for some things, that is, the supply of oil is already limited. Sometimes demand is managed (reduced) by a government limiting how much someone can buy rather than letting the price rise to exclude some buyers, but if by rationing you mean people can't get all the oil they want, it has always been rationed like anything of less than infinite supply. And as oil gets more expensive alternatives become more attractive. Even worse for oil, those alternatives are becoming dramatically cheaper.

Based on precedent we will shift from oil for economic reasons before we run out of it. A lot of oil will be left in the ground.