From the Chevy Volt website:
"With the extended range Volt, you don’t have to choose between electric or gas. You have the option of both. First is the advanced lithium–ion battery that allows you to drive gas-free for an EPA-estimated 38 miles. But Volt doesn’t stop there.
There’s also an onboard gas generator that produces electricity so you can travel a total of 380 miles on a full charge and full tank of gas†
. Now that’s long-range savings."