Tom,
I try not to be biased as I recognize that both have thier benefits and weaknesses. I agree that HSD has the advantage right now, and possibly a bit more than you think. I would love to have an HCH at our marathon next month so that we could compare the FE between the two on the same course. Maybe one of the HCH hypermilers can come for a visit.

Our test course is optimized for HSD, not IMA so I would expect the Prius to do much better than an HCH. But I suspect that on a course with higher speeds, say a fairly constant 45MPH, the HCH would shine due to lean burn. I would love to try my Prius on an HCH optimized course. If we test both technologies on both the HSD and the IMA optimized course, then we might be able to quantify the benefits of each technology in a more meaningful way.

OK you HCH hypermilers, lets come up with a HCH marathon!

It's a Wonderful Day for Science!