Here is a link to a calculator for Plug In cars that the EPA puts out. I don't know if the link has been posted yet. PHEV Fuel Economy
That's pretty cool. It doesn't seem too far off the mark. Interesting the 2013 Volt only saves $43 a year compared to my PIP with my particular mix of driving. The 2012 Volt costs about $67 more than the PIP. The CMAX Energi costs $251/yr more than the PIP. Thanks for posting it.