[MetroMPG]

Wow - talk about splitting hairs. I like it.

I'd say if the trailing face (as intended to be mounted) is sufficiently round, I'd mount it backwards.

That said, how tall is this thing? The turbulent boundary layer thickness at the rear of a normal sized car's roof is about 20-30 mm thick. So the shape of stuff that's entirely within that layer isn't going to matter as much as stuff that's sticking up into free flow.

That's my amateur opinion anyway.