Just thinking about this a bit further, there are different ways to improve the efficiency of a vehicle. There are three main forces that require power to overcome: air resistance, friction with the road and force needed to accelerate.
In theory you could have a way more aerodynamic vehicle, an actuated spoiler or something that optimised grip Vs efficiency, computer controlled acceleration so that it is done most efficiently.
I could see solar powered vehicles with no wall charging being possible, inside the limits of the amount of energy we get from the sun.