Here's an interesting car that Oldsmobile made in the mid 80's. They built it as a speed record breaker, but imagine the mileage you could get out of it at interstate speeds with an appropriate sized ICE and the right transmission ratios. They built this car in two versions, the long tail (boattail) and short tail (rear diffuser) that were otherwise identical. The longtail posted a speed of 278mph, the short tail 257mph, proving the superiority of the boattail in drag reduction. I couldn't find any data on its Cd, but it must have been very low. They were powered by a Quad 4 (4 cylinder). So the car companies know how to build low Cd car, they just choose not to put them into production. But just imagine the mileage that you could get out of a 4 seat version of this car at 70mph.