I agree that autonomous vehicles are an exciting and potentially transformational development; however, the outcome you describe is one possible scenario, and hardly the only one. It's far too early to make firm predictions about this set of technologies, especially with regard to the implications of their evemtual implementation. Despite progress by Google and others, opinions about how quickly and completely self-driving tech would be adopted are still divided, for good reasons.
You also appear to be making heroic assumptions about the potential impact on fuel demand. To reduce consumption as much as you suggest, either vehicle miles traveled would have to drop significantly, or autonomous vehicles would have be alternatively fueled, or ultra-efficient. There's no reason that autonomously driven cars wouldn't be conventionally fueled, especially in a lower-oil-price environment. (The cost of the self-driving technology by itself could raise car prices signficantly, and manufacturers would likely be looking for other ways to keep initial cost down, such as by avoiding costly batteries or hybridization.) In any case, our experience with many efficiency technologies is that usage actually increases--the so-called rebound effect.