At his point the numbers are practically arbitrary. Chevy wouldn't have to do much to come back and claim 370 mpg using the same methodology. Dropping 20 lbs off the weight of the vehicle might be more than enough to make up that difference.
Right now, numbers are just hearsay. We'll see what the EPA says once they get their new MPG rating system up for Series Hybrids. I think the Volt's gonna come out on top though... No way that other eyesore is superior