Chevy volt does 127mpg...

Huh?
It’s an electric car with a gas generator in the “trunk”, there is no direct gas drive like in a prius.

Edit:
Nevermind, looks like it’s more complex than I thought:
the gasoline engine can engage mechanically to the output from the transmission and assist both electric motors to drive the wheels, hence the Volt can also operate as a power-split or series-parallel hybrid when additional power is required.

Why is there a transmission if it’s driven by electric motors? Confused.