Useful? The Gen 2 Volt’s window sticker rating of 53 ev miles and 106 MPGe indicates the car’s fuel tank holds only 0.5 Ge (or, one-half gallon of electric energy equivalence) of fuel. Perhaps it would be informative if the MPGe gauge were limited to displaying mileage numbers only after you had driven a gallon’s worth of distance (i.e., after using two full charges of grid electricity, your mileage is x miles/Ge, after four full charges, after six full charges, etc.). How precise would you expect a mileage reading to be that attempted to calculate your "mileage" based on using 2-3 cups of fuel to drive ~13-20 miles?
Keep in mind that MPGe is based on electricity "from the wall," and does not attempt to deal with the impact of any additional electric fuel energy you add to the battery’s supply of grid power as you drive (i.e., regen).
If you drive until your Gen 2 battery is fully depleted and then multiply the achieved electric range by two, you have your MPGe mileage for that half-gallon of fuel. Combine the electric ranges you achieve using two full charges to get your MPGe mileage for one gallon of Ge used. Remember that getting 120 MPGe from a car rated at 106 MPGe is like getting 28.3 mpg from a gas car rated at 25 mpg. Is it particularly noteworthy?