WhiteCT200h2013

How does the onboard computer calculate mileage and speed. I drive 60 miles round trip to work 90 percent on hiways usually not at peak traffic, yet settings always calculates my average speed as 27 mile per hour. On the mpg, some weeks I get 47 mpg, other weeks/tanks settings tells me 42 mpg. Should I be using premium or mid range gas? I have been using Regular. Thank You