Sure robotic drivers will be masters of fuel efficiency, and quiet likely we'll end up sharing one car between many different people when that happens. But Jevon's paradox[0] suggests that we'll actually be using more fuel, not less. And it makes sense as well: driving will be so easy and not constrained by human factors (needing to sleep, eat, boredom with driving, being intoxicated etc) there are millions of reasons to have more cars doing more work.
Even ignoring environmental cost, the fuel cost to the individual would remove a lot of this usefulness (Why not commute 8 hours while you sleep, start your weekend on the beach the moment you wake up? Because it's so expensive!).
I pretty strongly believe that without fully electric cars, we'll never realize the benefits of fully robotic drivers.