This. What's displayed on the screen inside the car does not account for charging losses. The monthly OnStar report numbers do include estimated charging losses.The discrepancy of 20% is within the possibility of charging losses as well as temperature control on the battery pack during charging procedures. I don't believe the car takes into account those factors and only what is used to propel the car which could account for the difference. On a side note, 240KW/100 miles seems VERY excessive. Are you sure its not watts/mile or 24.0KW/100 miles?
Just don't use it all the time. Get a baseline measurement then get the kill-a-watt out of the circuit. Seems the continuous load has caused some to melt down and burn out.If you are charging using your L1 cord at home get a Kill a Watt power meter and keep track of your kWh usage and your EV miles.
That will give you true Wall to Wheels costs.
I would not say common, I would say rare but achievable. I would say most common efficiency for Gen 1 is averaging 3.5-4 miles/kWh.That's actually pretty common efficiency for a Volt. Careful driving in good weather actually gets it close to 5 miles per kWh (from battery, not wall).