The problem with that is that aerodynamic drag grows with the third power of speed, so going faster requires much more energy, even after accounting for slipstream benefits that a self-driving mechanism could exploit.

For some numbers, see https://www.quora.com/How-long-can-you-drive-a-Tesla-S-P90D-... . The top comment is about racing, but the one below it lists 300Wh/km at 160km/h and 380Wh/km at 180km/h. That's a quarter more energy per km (!) for going 20km/h faster.