As referenced in GM Claim 3, the Volts battery pack is rated at 16kWhrs but the Chevy Volt only uses 8kWhrs to protect the battery pack. Although it will obviously vary slightly, petrol has an energy content of 36.6 kWh/US gal. Thus, the battery pack is equivalent to 8/36.6=0.22 US gallons of petrol. Given that the Chevy Volt (from GM data) has a real mpg figure of 50mpg during the petrol part of the cycle, this is equivalent to saving 11 miles worth of petrol per journey.
None of the GM figures at this stage seem to be making sense or adding up, but significant differences between the claims and real world driving will soon show up when customers actually have one in the real world. This will do more harm than good and help to damage the public image of electric vehicles in general and the GM Chevy Volt in particular.
Energy content of petrol/gasoline: