Under normal, battery-charged conditions and under 70 MPH, the gasoline engine does not run at all.
This is not correct. The gas engine doesn't turn on at all when the car is in electric mode, even when driving over 70mph. When the car is in charge-sustaining mode, that is the battery is depleted, the gas engine is directly connected to the wheels under certain conditions from 30mph.
GM says:
"The engine WILL NOT turn on while the car is in electric driving mode (which for your trip two days ago approached 50 miles) – simply put, it is a full-performance battery electric vehicle.
Once the battery is depleted, the Volt’s gas-powered engine engages to create the power needed to extend the range of the vehicle several hundred additional miles. In extended range mode the Volt is powered by either a 1-motor series or 2-motor combined mode. The vehicle will select the most efficient mode for the driving condition: 1-motor series – for operations almost exclusively below 30 mph; 2-motor combined almost exclusively above 70 mph. At speeds in between 30-70 mph, the Volt will select the most optimally efficient drive mode amongst the two."