You were able to hold lean burn up to 91+mph? What RPM is that?
The Insight, in stock form and cool ambient air, struggles to hold lean burn at 68mph. VTEC kicks in around 73mph, which pretty much kills all hope of lean burn. It's clear the Insight was not designed for great fuel economy at >70mph.
If your 90mph lean burn was on flat ground, your engine can apparently make 20+HP in lean burn, whereas mine can't do more than 15HP.
Btw, AerocivicLB should join
Team Honda.