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It sure seems counterintuitive that something sticking up at the back of the car could reduce drag and improve mpg.
I cite an episode of Mythbusters where they tested whether it was more efficient to drive a pickup with the tail gate open or closed. They found that it was actually more efficient to drive tail gate closed. Which as you state here, is counter intuitive. They did further testing to see why this is and found that with the tail gate up it creates a small and beneficial vortex inside the bed of the truck. This bubble of turbulent air creates a cushion that the main bulk of air flowing over the truck rides over. In essence it creates a sort of virtual kamback in the bed of the truck. With a vertical, or near vertical spoiler on the back of a car you should have a similar phenomena, if only in a smaller scale.
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