I edited the co2 emisions of my Insight to a correct 127gpm (158gpm wells to wheels). Co2 emissions are directly proportional to what mpg you get. For a gasoline car you can use this formula to get a pretty close approximation of the co2 emissions of your hypermiled car: 50 / (your tank mpg) x 178= your tail pipe co2 emissions in Grams per mile (gpm). Then multiply by 1.246 for your "Wells to wheels" co2 emissions.
If you have a plug in car your emissions will be the ratio of your emissions on electricity and gas. You can look up your emissions at
Fuel Economy .The website will only do a predefined ratio of gas to electric use though. For example, a Chevy Volt get 98mpge on electricity for 119gpm "wtw"(on my New england grid 01201)/ and 37mpg on gas for 240gpm tail pipe (303gpm wells to wheels gas). The EPA assumes 66% EV miles, 44% gas miles for a combined 252gpm co2 wtw.
This dispels the plugin bias of not counting the electricity you use. A Chevy Volt may get an inflated 1,000mpg fossil fuel consumption,
but assuming you get the epa estimates and drive like the average person, your emissions will never be cleaner than 119 grams per mile on pure electricity, and like wise will only be as bad as 303 grams per miles. Infinite mpg does not get you infinitely small emissions. Electricity from the grid is part fossil fuel, so even if you chose to remain ignorant of it in your "mpg" calculations, Your greenhouse gas emissions do not lie and will always reflect it!