Mike Dabrowski had a cool display on his Insight. See the "Real time external MPG display" section about 1/3 way down the page.
Mikes Tips - MIMA Honda Insight Modified Integrated Motor Assist Tour De Sol
He has a great site btw. Off topic but check out his idea for a hitch mounted e-wheel in the "Projects" section. I've been giving this alot of thought lately...