Wouldn't the MPGe be directly related to actual miles driven on a full charge?QUOTE]
Sure, the MPGe is directly related to actual miles driven on a full charge, but you have to drive far enough to require recharging multiple times to use up one MPGe gallon. Your Volt’s electric fuel tank doesn’t hold much fuel.
The Wikipedia "Miles per gallon gasoline equivalent" article uses 33.7 kWh of wall-to-wheel electricity as the equivalent energy consumption unit for one gallon of gasoline.
For me, MPGe is far too large a number to have any meaningful application to the Volt. The usable power in the fully charged 2014 Volt battery, converted to MPGe measurements, has the same energy consumption amount contained in about 6.2 cups of gasoline. For the Gen 2, the battery hold the equivalent of about a half gallon. How precise can trip MPGe readings be when the full tank holds only a fraction of the fuel unit’s volume?
Your 2014 Volt’s gas tank holds 9.3 gallons of gas, your electric tank holds only 0.38 gallons of gas-equivalent. To drive the MPGe equivalent of 1 gallon of gas, you would need to drive on battery far enough to require three about full recharges. You would need to recharge a completely depleted battery ~24.5 times to put the gallon-equivalent of 9.3 gallons of gas into your electric fuel tank.
If you normally use up one gallon of gas in the time span needed to drive on battery far enough to fully deplete and recharge your battery three times, it’s easy to see why your lifetime MPGe might be in the midpoint between your MPGcs and your car’s MPGe ev rating.
Many of us do not often drive far enough in a day to fully deplete our battery, so have rarely driven our actual all-electric range using a full battery charge. If we don’t, we rarely confirm how accurately any given day’s full charge range estimate predicts the AER. Who among us has ever partially recharged the car multiple times to confirm we can actually meet or exceed the window sticker ratings of 94 (2012 Volt) or 98 (2015 Volt) or 106 (2016 Volt) "electric miles per 1 gallon gasoline equivalent?"