That proposal has never been made public, although it was believed to include running through the 11-mile test cycle repeatedly until the battery was depleted and then going through the cycle once more. The fuel consumption was then calculated based on the fuel used and the total miles driven. The 230 mpg figure drew a lot of criticism as being unrealistic and, for many drivers, it would be. For people who drive short distances and charge their car regularly, it could be a conservative figure.
With only six months to go until Volt production begins, the EPA still has not finalized the mileage calculation procedure for these vehicles. The results you get are highly dependent on the driving duty cycle and a panel discussion on the subject of ratings at the SAE Congress did little to clarify things. Savagian is confident that the EPA will finalize a test and labeling procedure soon. They'd better, because by law all new cars are required to carry an MPG sticker, and November will be here before we know it.
[Source: Wall Street Journal]