[brucepick]

I have no proof, no math.
But seeing the problem of rear drag as being caused by sucking a big vacuum behind the vehicle - if something were filling in that vacuum from behind, the "lead" car's fuel economy should improve. It doesn't have to create that vacuum all the time, so it gets a free ride, relatively speaking.

Of course, I think that trying to take advantage of this would be even more hazardous than tailgating. Please don't do this!! A square-front bus, following a close distance behind you, would likely do a good job of filling in your rear vacuum. But I wouldn't feel comfortable driving that way. I'd rather have the bus a hundred feet back.