Answer: A streamlined car will be more aerodynamic causing it be more fuel efficient.
Explanation: Aerodynamic-of or having a shape which reduces the drag from air moving past. A bulkier car will be less aerodynamic meaning that the air that hits it won't go around but push it back causing it to use more power which causes it to use more fuel. A streamlined car will allow air to flow around and over the car, a streamlined car will usually be less like a box and have more curves.