At highway speeds, the aerodynamic drag becomes a large part of the resistance your vehicle must burn gas to overcome. Thus clean, streamlined body shapes are crucially important for fuel economy. Especially so at higher speeds. In the case of electric vehicles, whose batteries carry limited amounts of energy, low air drag permits use of less energy per mile which in turn boosts their range. Large scale alterations in the body shape are probably not in the cards for most people, but a couple of things you can do to improve your streamlining are to remove any roof rack or overhead cargo box and keep your car clean and waxed. A clean, waxed surface slides through the air with less friction.