Cars with 6, 7 speed transmissions or CVTs already have a tall top gear at detriment to throttle response. The issue isn’t gearing at this point — efficiency is limited by air, drivetrain, and road drag.

You might be able to gain efficiency by making cars lighter (lesser crash safety if they crash less), but above 80 mph, drag is mostly from air resistance. Frontal area isn’t going to change much unless you start building 4-person autonomous motorbikes.

And if anything, autonomous cars will be even more bloated since people will want to stand up, eat, go to the toilet. Think of an 80s VW Microbus except scaled up 25% in all directions for fatties.