While you're driving, mid-air refueling style. Better make the link-up all in the same lane, though.

Autonomous cars will already line up together to get the slipstream efficiency; take it an extra step further and you could have a battery tanker-truck up front giving everyone in the convoy a charge and also making their travel more efficient.

And share the energy from regenerative breaking downhill, and much more.

At least, just like cyclists, they can rotate through who gets the front to equalize energy usage.