Your comparison with motorcycles is a bit flawed. Weight is the wrong thing to look at; at highway speeds drag is a major factor and motorcycles actually have

Your comparison with motorcycles is a bit flawed. Weight is the wrong thing to look at; at highway speeds drag is a major factor and motorcycles actually have higher drag coefficients than cars. You're also understating motorcycle fuel economy: I can easily get 4.5L/100km on my 600cc I4 doing 100km/h (although I more typically drive 130km/h and get 5L/100). I have friends who regularly do < 4L/100km in practice (on a 250, though).