Your only goal is efficiency?
Go low.
Cars are built high for practical reasons. Comfort, visibilty, safety.
Land speed racing has many ties to fuel efficiency. The better streamlined your car is, the less power you need to go a certain speed. In land speed racing, this translates to higher speeds with less power. For fuel efficiency, this means less fuel at the speed you drive.
One of the most innovative cars in racing is a series of cars called the Nebulous Theorem. They are built with nearly no ground clearance and extremely minimal frontal area. The NebII has set records at 360mph with a turbo 1.5L engine. (About 700hp). Power needed to overcome aero drag cubed is proportional to speed. So, if you assume 700hp to overcome drag at 360mph, only 3hp is needed at 60mph.
Pics can be found here:
Worlds Fastest 4 Cylinder Nebulous Theorem II