According to the New York Times, the current plan is to run car through each of the test cycles until the battery reaches the "depletion" point. From this data an equivalent mpg figure will be determined. The Volt will then be run on each cycle in charge sustaining mode to calculate fuel consumption. Researchers from the Argonne National Lab are currently studying real world driving habits to determine a "utility factor" for blending the two figures. It is expected that this procedure will yield a final sticker value that tops 100 mpg. Of course, because all of these choices are somewhat arbitrary, the bickering will surely continue for the foreseeable future.
[Source: New York Times]