I find I get the best mileage with my 2010 Insight if I stick to the 55MPH two lane state highways. I can easily get 50+ MPG in decent weather at 55-57MPH.
On the 65MPH interstates I am lucky if I can keep it at 45-48MPG @ 65-68MPH. And that is in good weather without too much of a headwind.
I rarely, almost never, go over 70MPH in either of my hybrids. In my eyes it really defeats the purpose. If you want to go 70-80+MPH
I recommend not even bothering with a hybrid.
But I suspect at speeds over 75+MPH
in decent weather you could expect 35-40MPG with a 2010-2011 Insight.