Nerys,
Since your Metro mass (weight) is close to the Insight, and your engine size if fairly close, and air drag due to Cd only slightly higher, then your straight line driving mileage should be better at a slower speed. If not, then like you mentioned earlier, there may be something to look at when the weather is warmer.
Just trying to help...
EDIT:
Almost forgot...
My daughter and I ScanGauged her Toyota Echo two years ago on a warm summer day (sounds good right now) and it easily got 55 mpg at 45 mph. She is a pretty good Hypermiler. Her mileage on the Interstate is 46 mpg.
I started Hypermiling the Cavalier back in the Spring of 2007 and got a 20% increase, and the same with an old crusty Honda 500. Both showed quite the improvement just by primarily, slowing down to 45 or so.
Jim.