My lifetime average for the car was just a hair under 51.6 mpg. My best (full) tank was 65.7 mpg. My speeds were much lower than yours though. Most of my commute is 45 mph with lights sprinkled between. The big hiccup with mine was my engine had pretty low compression where as your is brand spankin' new basically (I'm jealous).
I checked your garage but didn't see it on your current mods list. For aeromods I would highly suggest an air dam. It seemed to make a very noticeable difference with my Metro and it was super easy and cheap to do. At 65-70 mph it would probably make a great difference.
Here is my garage entry for my Metro. It has links to all of my mods as well as my build thread.
Details: Daox's Metro - 1999 Chevrolet Metro Fuel Economy - EcoModder.com