I may have to do a bit of backtracking on this FE issue
Right after I posted, I went out and ran a test at 60 MPH target, actually achieving 58 MPH average with the turn-around, and it sure looks like the Insight really hits a wall going from 55 MPH to 60 MPH. The FE dropped from around 100 MPG to about 90 MPG, using the same test course and driving technique. It looks like beyond 55 MPH is a wall of air for the Insight. I hope to make another test drive tomorrow morning to gain more "insight"
I have removed my lower grill block, for state inspection, but that wouldn't seem to explain the rather large difference in FE.
If this is indeed the case, then there is going to be a strong benefit from a good boattail.
The CleanMPG data that I referenced above is mostly by hypermilers, including mine ordinarily, though I do not pulse and glide. I simply drive fairly slow and am careful to maintain lean burn. The ordinary hybrid features like autostop and regeneration do help slightly.
More to follow