3.79 liter = 1 US gallon
1 liter of gas = 10 kw-h
1 US gallon = 37.9 kw-h
0.21 US gallon = 8 kw-h (energy available in the Volt batteries),
So, as the Volt will have a range of 40 miles, it will consume 40 miles for 0.21 US gallon, or an equivalent of 190 mpg in electric mode.
Why the consumption is as good as this is because the electric motor has an energy efficiency around 90%. As an internal combustion engine (ICE) has an energy efficiency around 20%, then the Volt would consume 190*20/90 = 42 mpg in ICE mode.