It is very hard to compare mpg ratings to determine which vehicles save more GHG and oil.   Confusion is the result.  And bad policy based on lobbyist exploitation of that confusion.

Average gallons per mile is a better way to state it.   If a plugin hybrid driver gets to work, shopping, and school and back without buying gas, then takes a weekend trip where the plugin battery range of 50 miles is exceeded by 150 miles.  

And the car gets 30 mpg in gasoline mode.  That is 5 gallons for a total 500 miles driven.

11 gallons of gas was saved.  Substuituted by 66 kwh of electricty, if that electricity came from renewables, it ought to get a 10 cent per kwh subsidy.  About a 60 cent government subsidy per gallon of gas and gallon's worth of GHG saved.

Reframing, it's the way to win in politics, the zen method common to Ghandi and Obama.  Change the perspective and backward thinking dissapears into the vanishing point.  Aum.

Become the clarifying reframe you want the world to become.