First, no one has tested a production level Volt with final calibration and final hardware. GM may discover some late change that needs to be made to fix a driveability issue or whatever, and the number would change.
Second, the EPA has not published the test method for plug-in hybrids. What assumptions did GM use to get the 230mpg number? Are they realistic? What about heat, or A/C, and electrical loads like the radio and headlamps?
For example, the FTP75 city driving cycle is 11 miles long. If the Volt has a 40 mile range running only on batteries, and starts fully charged, it will have infinite fuel economy for the city test. If it is tested with the batteries depleted, it will use its generator engine to move the vehicle, and will have some much higher fuel consumption (30mpg?).
I am not involved in SAE's committee working on plug-in hybrid fuel economy testing methods, but I suggest that the numbers be reported as follows:
- Line 1: All electric range, assuming some standard amount of heater and a/c usage, electrical loads, and wind resistance.
- Line 2: Gasoline only fuel economy (old fashioned FTP tests)
- Line 3: Combination, assuming X% battery driving and Y% battery depleted driving, for example, 50/50